My billsnake is suddenly all pissed off.

I've had a captive bred Stillwater Bullsnake for about a year now. When I got him he was about 2.5 ft long and now he is 5 ft long. I never had any problem holding him. I even did a talk on snakes with him and other people were able to hold him without any problem. Now he is hissing when I try and pick him up. He doesn't bite, but he wasn't even hissing before. He is eating fine and isn't sick. Has anyone else had this happen where their Bullsnake was friendly and suddenly became more aggressive? My bedroom is 78-80 because it is so hot out. Is it possible that is what is going on? Or are there any other possibilities? I keep him with two Rosy Boas but he has always been in the same cage as them.