irregular period or something else?

Today I saw some blood in my underwear that was pretty watery however I’ve already had my period a week and a half prior to this but it wasn’t light pink like implantation bleeding is said to be, it was dark red like a normal period is. Yes I’ve been sexually active and I have a very strong feeling that it could be my body adjusting to the hormones since I took a plan b after sex before my first period happened. I’ll probably still get a test anyways but I just wanted some insight on what is happening to my body right now from other women that might have experienced the same as me. 🙂