It doesn't have to be a particularly short explanation. I would start with something along the lines of:
"Hey (her name here), I realize that I've been acting kinda nervous or awkward around you, but sometimes I'm like that around cute girls, and I wanted to apologize for any stress I may have caused you. I think you're awesome and it seemed like you were trying to reach out to me and I didn't realize it at the time and I may have blown my chance at having you as a friend. If I did blow it, I'm sorry and forget I ever sent you this, but if I didn't then please let me know. I think it'd be really cool if we could hang out sometime, so give me a call if you want to go bowling or something (or not, cuz I would totally understand if you thought I was crazy). (Leave your phone# here). See you around.
It doesn't have to be anything all that brilliant or funny. Just don't stress over it. If she is cold-hearted enough not to respond to you being that vulnerable, then she is quite obviously NOT the greatest girl you've ever met. Let me know how it goes.