Boyfriend problems

So .... recently its about a week me and my boyfriend are having some problems i dont know whats wrong with him he has become so cold with me in which i havent done anything amd last week was his birthday and we had sex and stuff but before that i felt he is cold and again this week he even dont send me a single message we are both in a same university so i see him by accident everyday ...! I dont know what should i do the other day he told me he is sad for himself and he doesnt feel good! I dont want to lose him please give me some clues what should i do?