Too Early for hip problems in my 1.5yr old Tibetan Mastiff/Eurasier Male Dog?

I received Tater(my dog) when he was about 8-9 weeks old. There wasn't/hasn't been a problem with him getting groomed(God knows that dog sheds more fur than a stripper's clothes) and just recently I found that he won't let me touch or brush his hips too much. I experimented a little, and while I was brushing his tight hip/hind, he tried to snap at me, which he has never done before. I scolded him and he returned to sitting as if nothing had happened and I didn't brush in that area again for about five minutes, but when I started making my way down there, he barked like I had hurt him and jumped up. He wouldnt sit again for like two minutes so I had to calm him down by stroking his ears after I set the brush down. I ventured in the area(s) with my fingers and he seemed to be pretty alright by it. He was neutered about three months ago, but I don't know if that has anything to do with it. When I started lightly scratching near his pelvic bone he snapped at me again. If u can help, plz do