late period....

Charlotte
Hi everyone,
So three weeks ago today I had unprotected sex with someone. He didn't cum, and I'm not on the pill. As he didn't cum I didn't think anything of it, but my period is four days late now...
I had all my pms stuff (sore boobs, cramps) the week before my period was due, as usual, but then, nothing. I've still got a few pms things but still no bleeding at all. 
My periods haven't been regular for a couple of years, but it seems so odd that I had all that pms for no period at all. Does anyone have any ideas?