This is Pan and she is 9 months old. I took her in when she was less than a week old. She was a stray. Yesterday, I found her fur like this. I have no idea what it could be and I'm scared. Please help?
Don't panic. It's not good but it doesn't look life threatening to my untrained eyes and she should recover with proper care.
She needs to see a vet. It really is difficult to tell from this photo what is happening. I am not seeing any obvious open wounds which is a good thing and means it likely isn't an injury. It could be some external cause like fleas or ringworm. It could be a skin reaction to a bug bite or something she brushed against (allergies can develop at any time in life despite prior lack of reactions). It could be something internal that is hurting and she is licking it to try to make it feel better (like when a human puts a hand over a painful spot) which could be an allergy to food or something else. It could be a stress reaction and she is over licking it.
There are just way too many possibilities to guess at without getting a good look at it and the vet is your best bet. The vet will likely shave that area to get a good look at it to determine the best course of action from there. Good news would be that as long as everything else is good and there isn't bleeding this isn't an emergency vet situation but it is vet ASAP because that can't be comfortable for her and likely won't get better without some intervention. If she is not eating, starts bleeding, litter box changes, vomiting, diarrhea, lethargic then it is getting into emergency situation and needs care right away because it is more serious than the photo shows.
Hopefully your vet can prescribe a treatment and it gets better quickly. Crossing fingers.
Looks like it could possibly be a Hotspot. From what I understand this happened overnight? If so it probably is a Hotspot. These are extremely painful oozing sores and will only get worse without any medical attention. Please get her to the vet soon.
Alongside the above, it could also be ulceration from an animal bite, from another cat, perhaps. She might have laid on her back, in submission, and exposed her tummy to attack. Has she been spayed? It could be that she's in heat and rejected the advances of a male cat (cat mating is not a friendly event for the female). She needs veterinary treatment whatever the cause.