Can you cum without knowing it during the second time around?

I had unprotected sex with my girlfriend during her period. We thought it would be safer if i came once before we had sex because then i would last much longer, so she gave me a head. About ten minutes later we had sex. I was able to last a really long time. I never thought i came but for for some reason i lost my erection. Does that mean that i came? I dont think i have actually ever came on the second round. Normally i have to wait for a while. Could it be that i was just really tired and not turned on anymore? I know that because she was on her period the chances of pregnancy are reduced. Should i be concerned and tell her to get the day after pill? Thanks for the help.