Answer: Your co-worker may be having personal problems that she prefers not to discuss. If that seems likely, just remain friendly and wait for the situation to improve.
On the other hand, she may be sending a nonverbal message that she is upset with you. The psychological label for such foolishness is "passive aggressive behavior."
Passive aggressive people fear conflict, so instead of addressing issues directly, they act out their angry feelings. When someone asks what’s wrong, they invariably reply "nothing," thereby making it impossible to resolve the problem.
Because this is a game, the solution is to stop playing. First, make one final attempt at communication: "Mary, even though you say everything is okay, I still have a feeling that you're upset about something. Is there anything we need to talk about?"
If your colleague still insists that nothing is wrong, just say, with complete sincerity, "I'm really glad to hear that." Then take her at her word and ignore her silly pouting. If you continue to relate to her in a normal fashion, eventually she will either voice her concerns or quietly return to normal.
Want more information on the passive-aggressive game? See Dealing with Pouters & Sulkers.