So I talked and hit it off with a girl at college, but yesterday when she saw me, she walked right by me and didn't even say hi. I couldn't say hi as I only noticed her when she had come right next to me...
Should I say hi next time I see her?

Well, there could be many reasons that explain that behavior. You could have a talk with her, next time you meet, to clear that up.

Then you'll know better how to behave.