okay so me and my boyfriend had unprotected sex on the 17th of this month. It only lasted a couple seconds & he didn't cum or precum inside me at all. This app said I was supposed to get my period on the 20th and I still haven't gotten it 4 days later BUT my period isn't always regular & i never get it on the day they app tells me I'm supposed to.  Should I be worried about being pregnant? or am I just freaking out?