Is my co worker interested in me?

Over the last couple of weeks my female co worker has started to play fight with me alot more than she usually does. Its escalated so much that staff members are making playful comments about us. A previous co worker used to play fight with me, but not on this scale. It turned out that she liked me. So am i right to assume this co worker is the same?.
She asked one of the managers about relationships in the workplace and who would have to leave if she had one. A male member of staff told me it was directed at me. I jokingly made a comment to my female co worker, one that ive made before in which she found funny, but she got offended by it this time and didnt want to talk to me after that. I thought she may have been joking when asking about workplace relationships. But judging from her reaction she was being serious. Later on that day she began to play fight with me again and started to be overly nice to me, which she usually doesnt do.
My female co worker has been seeing a guy for a few months but refuses to call him her boyfriend when anyone asks if he is. Im going to assume she isnt sure what she wants from the relationship and she may be testing the waters with me. Am i correct?. I have noticed alot of girls that have shown interest in me before have been in relationships. Is this a common thing for women to do?. Any help would be appreciated.