Sounds like she is:
1. Urinating on her fur (which may be due to a urinary infection)and since it wet will leave marks when she sits.
2. Has some sort of uro-genital infection and puss leaking from the area is leaving marks.
3. Having incontinence problems probably due to infection.
4. Or just likes sitting in her litter box which would leave her butt wet and thus leaving marks where she sits.
