Is it normal to be dissed by someone you're usually friendly with?

A girl I work with recently blatantly ignored me when I greeted her. I know she saw me and heard me since the room is not terribly large, plus there were less than 10 people in the room. I said 'Hi!' clearly, and even waved. She looked at me for a few seconds as she was walking towards me to put her bags down, but didn't respond. However, when another friendly co-worker said hi to her just seconds later she said hi back. I haven't done anything to make her act this way. We're only aquainted, but she's never done that to me before. The only reason I could think of: I had a big birthday party and invited a lot of our mutual friends, but not her (because we are not that close!). I doubt that's why though. Any input?