Perfectly normal she is just warning you that she will bite. Call it a bluff... but do keep your eye on her if she is hissing and you need to do something around her. I notice that it happens more often for me when I am not around for a few days at a time.

The best thing you can do is rub her backside letting her know your there when your working around her. if your nervous about getting bit hold the hide or a cloth over her head while you do so before picking her up.

A few days ago i went in to change the water and check her paper. when i lifted her hide she hissed, but she kept doing it on the smaller snakes you may not catch them taking deep breaths between hisses. This as the first time she hissed at me and the first time i noticed the inflation and deflation...