Periods
Why do i feel low after my period?
Hormones swing sharply at the end of a cycle, and for some people that produces a period of low mood after bleeding stops. There is also a common link between PMS and PMDD, which is more severe and highly treatable. If the low mood lasts more than a couple of weeks or is getting worse, it needs a different answer.
What this does and does not tell you
This is general information, not a diagnosis. Only a clinician can examine you, take your history and run tests. The same symptom has different causes in different people.
If something is sudden, severe, or getting worse quickly, get medical advice rather than reading on.