Our two cats, who are both buddies and who have been fixed, have recently had a change in attitude that lasted for about 2 days. We let Simon out for a while but when I tried to get him back in he hissed at me. Then after getting into the house he and my female cat, Tiger, acted quite differently to each other. Simon would stalk and hiss at her even barring his teeth. He would voice his aggression to her also. This lasted about two days. Now they are touching noses again and acting like nothing happened. I was wondering if anyone had a clue as to what happened. Tiger is older than Simon and both have been fixed. However, Simon was neutered at a very young age. Could his tubes have grown back together and this be a sign of male maturity coming on. He is 1 year old. He even showed signs of aggression to me and my husband by hissing at us. Simon is usually a very, very, loving cat and this was an absolute shock to us. Any ideas?