It's probably the ringworm again. Your first instincts are good ones!
Cats are the most common source of ringworm in pets.
Be careful, you can catch it! Though humans have a ringworm that's all their own, it's pretty easy to pick up the pet's version too. Ringworm in humans is often known as ‘Athlete's foot' or ‘Jock itch'.
You can try to treat your pet with the same medication you'd use to treat yourself. Just be sure it says it treats "ringworm" on the label.
If this doesn't begin to resolve the problem in about a week, or if you notice that it's worsening, there are other symptoms such as pain, severe itching, going off their food/water, diarrhea or constipation, etc. - don't take chances - take them to the vet.
Two of the most usual meds veterinary dermatologists recommend are Itraconazole (Sporonox) and Griseofulvin (Fulvicin). Depending on the cat, health history and even age/size, one over the other may be preferred. They are not inexpensive options, both must be given with food (they are oral meds) and absolutely must be continued for the entire length of prescription even if the problem seems resolved earlier.
For more options on environmentally treating ringworm and topical treatments, check this link:
It's nothing you're doing wrong - this stuff just happens. You're doing all the RIGHT things in noticing it early and pursuing treatment. Good job!