Why does he behave like this?

the other week my college lecturer bashed his elbow into mine whilst in a crowd, quite hard, and then when i said ow he suddenly asked if i was alright, and i was rubbing my elbow and then i let go and he just reached out and held it and started rubbing it whilst staring at me, and didn't really let go when i tried to pull my arm away o_O he also is quite bi-polar around me, one day he will completely ignore me and help everyone in the room except me and the next day will always be staring at me or coming over to talk. but when he talks to me its always quite awkward and brief, and he looks at my desk the whole time, and he isn't that forthcoming. but when i watched him talk to other people in my group he was confident and maintained eye contact and whatnot and was eager with advice. and he sometimes is rude to me about the most random things, that aren't even worth getting annoyed about. what do you think his problem is :/