My boyfriend and I had unprotected sex right after my period and I took a generic plan b pill. We again had unprotected sex two days after, but he didn't cum inside me like before so we didn't get the pill. 5 days ago we had unprotected sex again and I took the plan b (actual plan b) this time. 3 days later I thought I got my period because I'm supposed to get it 4 days from then, so I put a tampon in. I took it out and there was dark brown blood on it. I figured it was my period but it's very light. I only see the dark blood when I take a tampon out. Is it the effects of plan b? Or can it be implantation bleeding?