Hey there. Welcome. Health anxiety is such a pain I have it sometimes myself. I understand how you feel and this is actually something well studied. I think we naturally can worry about something health related, that's normal. We all do. but if it preoccupies our minds to the point we have trouble functioning, then it's time to talk to our doctor about it. Treating general anxiety can be a great help. Have you ever considered doing that? What things to you do to keep your health anxiety in check?
There are no doctors on here so we can't give you any medical expertise. The only thing I can think of is, if this was the only time this happened, you might have gotten your period a bit early. When you say vigorous sex, we can't know what you mean, but if you mean you like it rough, you might be irritating yourself. It doesn't sound likely, but I suppose it's possible. If this were the only time, to me, the fact it kept bleeding as it does when you get your period for a few days indicates it was your period. But if it happens when you're nowhere near getting your period, then that definitely isn't it and you do need to see a specialist to see if something is going on in there.