What is his game??????????

A male friend of mine is acting strange with me. He talks to everyone else but me, flirts with other women while I am talking to him (which is fine except we have a history and I would never do it to him) and generally seems to be isolating me. Then today I was talking to this older lady in the corridor admiring her clothes. 5 minutes later he comes down the corridor singing and completely ignoring me, he starts admiring what she is wearing. I say yeah, its lovely isn't it and he acts like I am not there. So I just walked off and said I will leave you to it. I really hurt and confused by this behaviour. What is his motive do you think or have you ever done/ seen anything like this before?