On the pill but having a scare!!

So i had protected sex about 3 days ago (and for the first time) and i was on the pill but have missed a few from this month's pack. it's been 3 days and ever since the sex i've been experiencing cramping and loss of appetite. i don't want to think i'm pregnant but more of why i'm cramping so much. today i started my placebo pills and my period doesn't start until around the 4th placebo pill but wanted to know if anyone has experienced this? please give me some feedback!