My dad's cat seems to have a constant mucky bum

[YoniMatopoeia]

It looks matted with poo. None of my cats were like this. He has had to stop the cat sleeping in his room because she was leaving behind little lumps/flakes of cat shit in his bed.

Any ideas on why this would be? I have always thought of cats as quite clean in that department

I have suggested that he worm her (which he has done)

I'd take her to the vets, doesn't sound right to me at all.

Poor cat can't be comfortable either, if she tries to clean it herself that's awful for her. Imagine eating your own poo.