Pill, unprotected sex, and cramps but no blood on period

I've been on the pill for two months now, I had my first 7 day withdrawal bleed and then started again when I was supposed to. When I was 3 days into using my pill again I had unprotected sex, now my period is 3 days late but I've got all the normal pain. Backache, stomach cramps but I still haven't bleed......has this happened to anyone else?