Missed period, still a virgin. What's wrong?

I'm 19 years old, and my period is currently 21 days late. My period has always been pretty constant and very rarely has been late. However, it has never been this late. I am a virgin, and I haven't been in contact with men in suck a manner that it would lead to pregnancy or anything. I haven't felt many other physical changes, expect for my nipples being a little sore and sensitive for the past few days. Other than that, I feel a-okay.