Smelly Cat

Hi!
I'm a gynecology med student, and cats are not my forte!
My cat, a 4 year old neutered tiger cat, has a vile smell coming from her. It first happened when she went outside after it rained. She came back inside, wet, and immediately began to lick her behind under my bed. She was biting and gnawing at her butt for about 4 hours, not letting anyone come near her. Now, if anyone goes behind her, or scratches the area around her tail or lower back, she begins to growl and swat at them. The smell smells like a mixture of feces, urine, and mold, but when you look at her behind, there is nothing there.
Is it a skin infection or a GI problem?
Thanks!
Also...while I'm here, I might as well ask my other question, as well.
Sometimes I will see long white circular things coming out of her anus. Are they worms? They seem to wiggle out and have an almost flat head.
Thank you!

[Dan Sims, DVM]

Cats have anal sacs just lateral to their anus that can impact and abscess.and emit a foul odor.An abscess due to a cat fight may result in a foul odor also.Your Veterinarian will help you as this can be a painful issue for your cat.

[CurleyGirl7]

Thanks for your help!
I made an appointment to see the vet today.