Complex situation and need help please PLAN B

So I am currently 4 days late for my period and yesterday night me and my so had sex and we used a condom but so how it slipped of and he wasn't to sure when I came off and he doesn't know if he came inside the condom or me so today I tock plan b just to be safe...but I know it make your period late so when should I expect my period...and I don't know why it late it been. Irregular since August and we used a condom all the time Nd. He didn't I'm inside me...so again when should I expect my next period and how would I know it late????