Light period - opinion needed

Kiay

Ever since I received my monthly it’s always been the same. To the point where I can tell what kind of flow I’m going to have each day. Recently I was sexually active earlier in the month. Later on in the same month I received what should have been my period ( still not sure what it was). It was light to the point where I couldn’t fill up a panty liner for each day. It was dark red or brown. And it seems as if it was mixed with discharge to clean it up.

So I decided to take a test a few days after it stopped and it was negative.

I’ve read up some things, so I don’t think it’s pregnancy.

I’m going to go see an OBGYN

Have this ever happened to anyone?