Is this something to be wary of in a guy?

a new coworker was very nice and polite with good first impression.but when i tried to give my opinion on the job, he turned very ice cold to me. doesnt smile at me nor even look at me anymore.he always says subtly how crappy the other workers are.however, with the students he can switch on being nice again, then when he has to talk to me, is cold as ice again.when we are in the room together with the students,he doesnt even acknowledge my presents.sorry for bad english