Why does my period feel different from last month?

We often expect that our menstrual cycle will follow a predictable rhythm, but this isn’t always the case. The flow, the timing, the cramps, and the emotions can feel different from one month to the next.

This variability can be confusing, or just a total inconvenience. So let’s explore what the science says about why periods can vary.

The HPO axis

Your menstrual cycle is governed by the hypothalamic-pituitary-ovarian (HPO) axis. It’s the system where your brain (specifically the hypothalamus and pituitary gland) and your ovaries work together by sending hormonal signals back and forth. Think of it as a dynamic feedback loop between your brain and reproductive system. It’s sensitive and deeply responsive to your internal state and external environment.

In an ideal month, the HPO axis sends signals in a tight choreography: the hypothalamus releases GnRH (gonadotropin-releasing hormone), triggering the pituitary to release FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone), which in turn prompt the ovaries to grow a follicle and ovulate. If ovulation occurs, progesterone rises, leading to a bleed roughly two weeks later.

Research shows it’s highly sensitive to factors like stress, sleep, nutrition, and inflammation. Even subtle shifts in your daily rhythm can alter your cycle (ever had a period turn up early/late when you go on holiday?) One month, ovulation might happen a little later; another, not at all. One month, the endometrial lining may build up more making your period heavier; another, less so. (Dhodi, 2025) (Lawson, 2011) (Kapper, 2024)

The external can change the internal

Stress is one of the most well-documented disruptors of the menstrual cycle. High cortisol levels can suppress the release of GnRH, which can lead to delayed ovulation, a shorter luteal phase, or even stop ovulation completely (Loucks, 2004). This can result in periods arriving earlier or later than usual, or skipping a period.

Sleep also plays a crucial role. Irregular or inadequate sleep impacts melatonin and cortisol regulation, which in turn influences the timing of ovulation. A 2023 study published in BMC Women’s Health found that women with irregular sleep patterns experienced more variation in their cycle length and reported higher rates of premenstrual symptoms.

Nutrition and energy availability play a big part too. When your body isn’t getting enough fuel, whether from under-eating, overtraining, or ongoing stress, the brain may reduce the hormone signals that support ovulation. This is known as Functional Hypothalamic Amenorrhea (FHA). While FHA is often linked to missed periods, research shows that even milder disruptions in energy balance can affect how consistently you ovulate. And when ovulation becomes irregular, you can still have periods but they can feel different, with changes in flow, pain, or mood (Gordon, 2024, & Rasouli, 2023).

Naturally cycling vs hormonal contraception

People who are naturally cycling tend to experience more noticeable month-to-month variation because their bodies are producing fluctuating levels of estrogen and progesterone in real time.

In contrast, those on most forms of hormonal contraception don’t typically ovulate - especially with combined oral contraceptives like the pill. Instead, synthetic hormones suppress the HPO axis. The bleeding that occurs during the placebo week isn’t a true period but a withdrawal bleed from the drop in synthetic hormone levels. It tends to be lighter and more predictable, but that doesn’t mean it's symptom-free.

Studies have shown that hormonal contraceptives can still produce variability in emotional and physical symptoms, depending on the type of progestin used, the individual’s hormone sensitivity, and whether they skip or extend their cycles (Berenson, 2008). Some people feel emotionally stable on contraception; others feel more prone to mood shifts or fatigue. Importantly, many people still report fluctuations that feel cyclical, even without ovulating.

When you live with a hormonal condition

The picture becomes even more complex for those living with conditions such as PCOS (polycystic ovary syndrome), endometriosis, or PMDD (premenstrual dysphoric disorder).

In PCOS, ovulation may not occur consistently and when it does, it can be delayed. This causes unpredictable cycle lengths and highly variable bleeding, from spotting to heavy flows and clots. Elevated androgens and insulin resistance in PCOS also contribute to mood instability and inflammation, which can influence how each period feels (NIH PCOS Guidelines, 2023).

Endometriosis is characterised by inflammation, nerve sensitization, and lesions that can respond differently from month to month. Some months, pelvic pain is localized and more manageable. Other months, it radiates to the back, bowel, or bladder. This variability isn’t always tied to hormonal shifts — it can also reflect flare-ups driven by immune activity, diet, or stress (Maddern, 2020).

In PMDD, the issue isn’t hormone levels, but sensitivity to normal hormonal changes throughout the menstrual cycle. Some months, the brain is more reactive to these shifts, and symptoms like mood crashes, rage, or despair can spike. Other months, the response may be more muted, depending on stress, sleep, and neuroinflammatory load (Keijser, 2025).

Tracking to learn your unique cycle

Samphire helps you make sense of your cycle — not just track it. You can log symptoms, emotions, sleep, stress, and even create your own custom trackables.

Over time, you’ll start spotting patterns you can actually work with, so you can plan, prepare, and respond with a bit more clarity (and a lot more self-compassion).

And if PMS and period cramps are a pattern you’re experiencing, Nettle™ can help to alleviate those symptoms and give you a much more manageable and predictable period.