Help with cat

I am not the primary care taker of this cat. My moms cat was laying here and I notice a small dark spot where she was sitting coming from her butt. He butt is also a bit wet. Just wondering if I should be concered she does miss my mom who is away till wed. the cat has no tail but this has not been a problem in the past the cat is 7 years old. I dont really have the money to take her to the wet but if it has to be done i will.