Contact while on period, little precum, could I be pregnant?

So I'm totally confused right now. I'm even confused on if I should be worried about pregnancy in this case. So on the 23rd of April I gave my boyfriend oral and he ejaculated a tiny amount. Then he rubbed himself with precum (which I am aware since he came that it would have sperm in it) on my underwear. He wasn't wearing underwear. I'm not sure if he got any on my underwear at all since I wasn't directly on the head of his penis. I was ON MY PERIOD at the time and it was my 2nd day of being on it. I had a tampon in as well. The problem is my period was weeks late that month. So I'm wondering if I could have ovulated on that period and if sperm could get through my underwear, passed my tampon, and into my Fallopian tube? I had brown discharge 3 days after my period and am now having watery clear discharge. My cramping is severe and I'm so confused if this was implantation and I could be pregnant or if this is something more serious...my cramping is on one side mainly and it hurts to the point of not being able to move.