Why Did My Period Suddenly Become Irregular?
Written with Autumn — The Change’s on-device AI perimenopause coach.
The short answer
Irregular periods are often one of the first signs of perimenopause, but they are not a diagnosis by themselves. Pregnancy, hormonal contraception, stress, weight change, thyroid problems and other health conditions can also change when bleeding arrives.
Irregular does not mean one particular thing
A period can arrive earlier, later, be skipped, last a different number of days or change in flow. One unusual cycle does not tell you the cause. A pattern over time gives a clinician more to work with.
How perimenopause may fit
The NHS says periods may become irregular, lighter or heavier during perimenopause. The transition can make timing harder to predict, but that does not mean every irregular bleed should be written off as hormones.
Other explanations to remember
- Take a pregnancy test if pregnancy is possible, even if your cycles are changing.
- Hormonal contraception can make bleeding irregular or stop it altogether.
- Stress, marked weight change, heavy exercise and some medicines can affect periods.
- Thyroid conditions and other health problems can also change menstrual timing.
What to do next
Notice the dates, flow, pain and any bleeding between periods or after sex. You can keep this as a simple calendar note or say it aloud to Autumn. The goal is a usable account of what changed, not a perfect record.
When a clinician should check it
Arrange an appointment if irregular periods continue, are accompanied by heavy bleeding or pain, or are affecting your life. Any bleeding after 12 months without a period needs medical assessment. Urgent advice is appropriate if heavy bleeding makes you faint, weak or unwell.
Related reading
- Why Are My Periods Getting Closer Together in My 40s?
- Why Are My Periods Suddenly So Heavy in My 40s?
- Why Am I Suddenly So Anxious in My 40s?
Want to see the pattern more clearly?
Autumn can help you describe the change in your cycles and the symptoms around them so you can prepare for a healthcare conversation. Autumn is the private, on-device AI coach inside The Change—not a clinician.
Sources & evidence
- Symptoms of menopause and perimenopause — NHS
- Missed or late periods — NHS
- What are menopause and perimenopause? — NHS
Sources checked: 18 September 2026.