If you're confident it's hissing and not distressed breathing...then my guess would be that she's very stressed. Especially if the hissing is coming when you're not messing with her at all. Is she in a glass tank? If so, I'd consider covering the sides with something opaque. I'd go so far as to completely cover three sides...and then put a 6"-8" strip of the same material along the bottom of the front side as well. That will make it so she can't see out without deliberately looking...but you can still see in easily enough. I'd also consider putting in additional hides or other decor she can use for cover so she doesn't feel exposed. It all depends on what kind of set-up she's in....but definitely look at it with an eye toward making her feel more snug and secure.

(FYI...if you choose to add some fake vines or other plants as ground cover for her to hide in...be sure to check it over VERY carefully for any closed loops of plastic or thread and cut those off completely!)