Need intelligent answers so in this category?

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. When I am around him he acts all confident with others, talking loudly, getting attention from others. Yesterday I was speaking to him and when we finished talking he turned away and when he should have said goodbye to me, he smiled deeply at another girl, then glanced back at me when I was leaving .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?