My cat used to get sores on the base of her tail from flea allergy dermatitis. She was basically allergic to flea bites. Didn't ever have visible fleas but one bite and she'd get these skin breakouts. Try a topical flea treatment like Front Line or Revolution (buy it from the vet, not the store, as the vet sells the only safe ones) on her head, and then ointment like Neosporin or Polysporin on her sores. If you cannot even afford to do that, clean her quarters as best you can for fleas (vacuum, wash bedding, etc.) and try cleaning her skin with white vinegar diluted in a lot of water. If it's flea allergy dermatitis, you have to get rid of the fleas for her to get better overall.
Annie is correct could well be flea bite dermatitis, although the cat doesn’t necessarily have to have fleas to get the condition...just a former contact with them and the condition can develop, can also be what’s called Stud Tail...even in females. Check out this site. https://www.petmd.com/cat/conditions/skin/c_ct_stud_tail_supracaudal_gland_hyperplasia