Could it be implantation bleeding and when should I test?

I am on birth control, but it is our only kind of protection. I started my period on Friday of last week. I finished on Monday and my fiance and I had . After we finished, I noticed some slight bleeding. I just umed it was because I was still coming off my period. It stopped a few hours later. Tuesday night, we had again. Again there was spotting. It continued (not even enough to show up on a panty liner) through Wednesday night. I started my active bc pills on Sunday night, so I shouldn't be spotting. Could it be implantation bleeding? When would I be able to test? Or should I just wait and see if I have my period next month? I've heard that taking bc while pregnant can cause complications, so I am very wary of continuing it without knowing if it could hurt my possible child.