Tell the vets exactly what you said here. Maybe print this and bring it with so you remember everything.

You said she is overweight but how old is she?

I think she's about 8 years old. She was abandoned by a neighbor, so I'm not exactly sure; she could be much older.

She weighs about 13 pounds now, down 2 from last January. I've tried a lot of different kinds of quality cat food to help her lose the weight, but she's a tough customer to satisfy.

Sounds exactly like my cat, back when she was fat. She always had poo on her rear-end, because she couldn't reach it to clean it; butt-skooching was a common activity of hers (joy). And a couple of times, it got all nasty and wet and yucky back there...ended up being a UTI both times.

If you've gotten her down 2 pounds over a year, then you're doing great. Just make sure you're not overdoing it -- if cats lose weight too rapidly, then they can get Hepatic Lipidosis...which I learned to my great dismay. My cat almost died from it. So, be happy with the weight loss you're getting, and if you manage to get a pound off a year, then you're doing really good -- by the time she hits around 12 pounds or so, I bet she can lick her own behind, and no more rear end crusties :)

I actually learned about the risk of hepatic lipidosis on this forum (thank you, GardenWeb!) so I've been careful about not cutting her back too much. (And of course, she simply eats the other cats food if she feels too aggrieved.)

She's in a lot better shape than she used to be, and she's very clever about propping herself against the furniture, so she can bend even more. But as with humans, the best approach is to increase exercise. Luckily she's still kittenish enough to like to play.

I've had cats that got UTI's before, so that doesn't seem as scary as what I was imagining. But I've made a list of bullet points with symptoms and changes to her environment, in case the cause isn't as simple as it seems.

i'm a dog person so this may be way off. But have you tried cleaning your cats rear everytime it goes potty? I clean my dogs rear and girly parts everytime she goes to the bathroom. I use regular baby wipes.

She really freaks out about the wipes. (I even tried warming them first!) Oddly enough, I think she tolerates the sitz baths much better than the wipes. She panics when she sees me get out the package, but she's quite calm about the tub. Even leans in to see what I'm doing when I'm taking a bath.

Good news from the vet, and it turns out there was very little to worry about. They did two fecal tests and ruled out any kind of bacteria, infection, or parasites. They think she simply got diarhhea from the food change, and enough of that got trapped to give her a really bad case of diaper rash!

They shaved the fur pretty close, and it does look like an angry rash. We have a little tube of ointment for the next few days, some metronidazole, and a probiotic to help her recover from the diarhhea.

They said she was an excellent patient, but she's happily home, explaining to the other cat why her butt looks so strange....

Thanks for the advice!

My dog used to freak out over the wipes too...I'm not sure how trainable cats are....but my dog knows she is not allowed inside until she is cleaned. She still does not like it and occasionally runs from it still but she sleeps on my bed so she will be cleaned :-) Its a little difficult with her because she has a tail that she will clamp down when she wants to be wicked. I am much more determined than she is though.