During Sex, The Condom Broke But We Didn't Realize Till

During sex, the condom broke but we didn't realize till after my boyfriend had finished inside me. We ran to the store and bought Plan B and I took it within an hour of having sex. Right after we found out the condom broke, I ran to the bathroom to try to clean myself out as best as I could. I knew my period was suppose to start any day and when I was cleaning myself up I noticed there was a little blood, so I figured that was my period starting. After taking Plan B however, I didn't get my period and now (three days later) I had a bit of spotting but my period hasn't seemed to have started. I am getting cramps and headaches and breast tenderness though. Do you think this is just a very light period caused by the Plan B or implantation spotting?