Overreacting?

Here's my latest concern 🙄:

We had unprotected sex 3.5 weeks ago during my fertile period (because we were both drunken fools 😷), then I got my period a week early which never happens. It was relatively heavy, but didn't last nearly as long as my periods usually do and I had virtually no symptoms (other than bleeding). Now 1.5 weeks after my period, I'm struggling with intense nausea and I'm exhausted- really that's the only reason I'm concerned.

Is there any chance that this was heavy implantation bleeding? Has anyone else had heavy IB? 

I know very well that y'all can't tell me if I'm pregnant or not, but do tell me if I'm overreacting 😭