What is wrong with my kitten?

Question
Normally my kitten is hyper, running around and everything. But starting today I picked her up and she started hissing at me. My boyfriends father picked her up and shye did the same thing. We figured out when we just touch her stomach it hurts her. She meows and makes a sound like its hard for her to breath. She is in heat, could this have something to do with it? And my male cat, who is fixed, seems to get a little rough wtih her every now and then but nothing serious. I'm really worried about her.

Answer
Hi Sommer.  Estrus causes hormonal changes that can make the mammary area a little tender, but for her to react so strongly seems abnormal.  This sort of reaction sounds more consistent with an injury or some pretty severe inflammation that may be associated with an illness.  Females of reproductive age are at risk for an infection of the uterus called pyometra, which is life-threatening, and a painful abdomen is a symptom.  If she swallowed something she shouldn't have, this could cause belly pain, too.  Maybe even if she tried to squeeze through a door behind someone but didn't quite make it, she could've had her ribs cracked.  A bad stomach virus is always a possibility.  And especially since it appears that her breathing might be involved, a vet should really look her over to determine whether she's sick or just hormonal.  More often than not, a mood change is related to a physical illness.