5 days late

I've been on the pill for about a year now and my periods never been late for more than a day or two, but this month my period is 5 days late. I haven't had intercourse since way before I got my last period, so I don't really know what's going on. Can anybody help me maybe understand why this might be happening? It seems very improbable that I could be pregnant but the worrywart in me doesn't let the thought slip away.