Possible Pregnancy?

I know you guys are probably sick of questions like these but I need some advice. I stopped using birth control last month and a week later I got my period. It was pretty typical, Like the ones I used to have before getting on the pill and nothing was out of the ordinary. I had quite a bit of unprotected sex during my fertile week and I'd be lying if I said it wasn't possible I'm pregnant, but two days ago I got my period (4 days before my period was due). This period, however, has been substantially lighter than my norm (though I wouldn't have considered it "light" as I am a pretty heavy bleeder) and today is down to light spotting. It was heavier than what I thought implantation bleeding could be but because it's about to be over so soon I'm a bit nervous. Do you think it could have something to do with getting off of birth control or should I think about picking up a pregnancy test? Has anyone else had implantation bleeding that was more than just light spotting? I didn't think it was possible.