Late period or pregnant?

Tayla
I've been taking the pill for a while now. I'm not perfect with it time wise but I take it everyday. 
My period is rarely ever late but it's late right now. I had unprotected sex just over 3 weeks ago and haven't missed a pill in that time but my period is now 4 days late. 
I may be paranoid. But just very worried :| 
Any advice or words of reassurance would be very helpful!