Pregnant???

I had unprotected sex with my boyfriend a week and a half ago and he ejaculated in me a little bit. Since we were going to get a Plan B pill the next day we decided for him to just go ahead and unload inside of me (even though that was very stupid). I don’t know if this makes a difference, but he pushed all the way inside of me when he unloaded. I am on the pill, but I hadn’t been taking it like I should. I took the Plan B pill about 22-23 hours after this happened. I had spotting about 24 hours after we had sex. I did have my period, but it lasted 2 1/2 days and was very light. My period normally lasts 5-7 days and it goes from heavy to medium, but it’s never thus light and thus short. What’s the chance that it was implantation bleeding? What’s the chance that the Plan B will be effective?