I had sex and then my period came and lasted for nine days - why?
I think he went too far and my period was already a week late.
There are several different possibilities here, and I would definitely recommend that you make an appointment to discuss this with your primary care doctor or your OB GYN doctor, especially if you are still having some bleeding or discomfort. Although it is possible to have some vaginal discomfort or spotting after vigorous sexual intercourse, this would not typically cause nine days of heavy menstrual-like blood flow, so I don't think this is what is going on. A more important clue I think is the fact that your period was already a week late when you had sex. This could mean a few different things, including that you were already pregnant and that the subsequent heavy bleeding was a sign of a very early miscarriage. Your period could also have been late for other reasons, especially if you have a history of irregular periods; for example, one very common causes of irregular periods is polycystic ovarian syndrome. Start by setting up a visit with your doctor at your earliest convenience. They can perform an examination and help determine whether this is an issue that needs any further workup, which it might if it is a problem that occurs more than once.
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