My wife and I live in a small flat along with our neutered male British Shorthair. Over the last month, we both traveled quite a bit and were away from the flat for more than 15 days. During this time, a friend of ours took care of our cat, simply cleaning out the litter box and feeding him. After we returned, I tried to clean out his litter box and he suddenly hissed and growled at me and tried to attack me. I stopped and waited until he calmed down. After about 10 minutes, when I was sitting a bit away from him, he approached me and let me pet him as if nothing unusual had happened. I am now afraid of touching his litter box and have absolutely no idea what caused him to get angry.
Any ideas what I can do?