Spotting - Pregnancy or side effect of Plan B?

I started my around the 5th and then around the 8th or 9th I ended my period and on the 10th I had oral sex but touched myself down there on the outside after we had finished and I think that I may have had a small amount of semen on my hands, but I was not worried. on the 11th I had oral sex again but this time was more worried and proceeded to Plan B the same day. On the 15th I had actual sexual intercourse and used a condom, and today (July 16th) I have had spotting. It is very light, but a deep red. Is this implantation bleeding? Could I be pregnant?
Some other contributing factors may be that the sex was a bit rough on the 15th and also I have been under a lot of mental stress. And also I have taken Plan B before, but it was close to my period and before my period I normally have spotting, and the last time I took Plan B I did experience spotting before my period, but it could have been a symptom of my period or it could have been a side effect of Plan B.
I know that spotting could also be a side effect of Plan B, so I was wondering if it was more likely that my spotting is an occurring because of Plan B or is it because of implantation?
Thank You, anything helps.