Playing hard to get, why is she doing this?

Theres this girl at my work and i think she likes me. She turn round and eyed me up once but the only thing is that she has not been very nice to me. the other day she said goodbye to everyone but didnt say goodbye to me. also i tried to add her on facebook but she hasnt accepted yet. She has asked me if im alright a few times and always smiles when she talks to me. . I dont understand whys shes doing this we work next to each other so i dont see why. what should i do?