My cycle is 3 days late??!

I was supposed to start my cycle on Christmas, but it's still not here. My last cycle was a day late last month so I tried not to worry, but it's been 3 days. My ex and I had sex like a week before my last period and I read that you can still have your last cycle & still be pregnant. Should I take a test? I'm 18, and I've been having my period since I was 13 & this has never happened.