My boyfriend and I had sex last night, he was wearing a condom but it slipped off inside me and we didn't know till after he cam. This morning I'm having minor cramps.. could that be implantation occurring? (I never have cramps after sex)

It doesn't happen that quickly. It takes 3-5 days for the sperm to travel up the vaginal canal, through the uterus, and up the fallopian tubes before it reaches the egg. Then it takes another 3-5 days for the fertilized egg to reach the uterus where implantation occurs.

Agree, it may be from the condom coming off, or a dozen other things, including worry.

At this point you will just have to do a pregnancy test if you fail to have your next period.