Bled during sex for the first time?

Hello :) recently my periods have been irregular as I came off of the contraceptive injection about 2/3 months ago. It seemed that my periods began becoming regular again as my last one came 3 weeks after the one before that when they have been coming whenever they liked. However, I came off of my period 10 days ago and whiles having sex with my partner (who I've been with for a year) I bled during sex..

Could it be that I've come on my period early again? It was a rather generous amount, so I don't know if it could be spotting because I'm pregnant..

I've been checked for sti's before getting with my partner, so the only way I could get one is if he has cheated on me and passed it on.

Any ideas what this might be?

Oh and I'm not bleeding as much as I did last night, if that helps. Thanks