Wonder bottom in cat?

Hello, my cat has been acting rather lethargic for the past two days, and I noticed that all the fur on her bottom is gone and it's sore. I wanted to take her to the vet tomorrow, but does anyone know what this is?

I gave her flea medication a few days ago, which is actually only herbal, but she kept licking it off her fur and I wonder if that could be related to it.

By the way, she is not fat, it is just the fur around her that she always pulls out and yes, she doesn't have a tail anymore