Implantation bleeding? (posting in different area)

Mila
I realized this should probably in sexplanation? 
Hey guys.. so my boyfriend and I made a dumb decision. We had sex without a condom. I was on my last day of my period it was on the 12th and ending on the 15th, when we had sex. My period usually last for 4 days. Heavy at first and then gets lighter as the days go on.
We had sex and he pulled out of me. I remember the next day I saw like some brown spots and thought that my period was still going, but the day after that when I was at work, I went to the bathroom and when I wiped there was a good amount of fresh blood. It wasn't there in the morning and wasn't there at night.
I freaked out and got Plan B since it had been less than 48 hours since we had sex. Even though I read it's use less after implantation. 
This is the first time this has ever happened  to me and I'm very stressed out. Could it had been a longer period? Or could I be pregnant?
I also had been on a new diet for a month before my period started, could that have altered it?