Period or IB?

I had unprotected sex three weeks from tomorrow, I'm 4 days late as of today, all though, today I started getting a light spotting type deal and now it's a very very very light period type deal, but it's like pinkish brown and not red, and it's like watery with discharge, could it be IB? Because I have a short cycle and was ovulating when I had unprotected sex, I've never missed a period before and it usually never starts out like this? I read that women start IB around three weeks after conceiving ? I've had mild cramps and nausea with sore swollen boobs about a week after sex, which was 2 weeks before my missed period, way too early for pms. Could i be pregnant?