3yr old female itching her fur off?

My eldest Minttu is 3 now and a month ago her fur around her neck was getting thinner. Atfirst I thought "Old age?", then it became more noticable, I quickly checked everyone over for mites and gave them all bathes anyway, just in case. But now, she's bald on her shoulders and now she's starting on the sides of her neck. Her toenails are not any longer than before (infact, they seem more dull than they were in her younger days). She has a tumor between her leg and her vagina, which she's also licked most of the fur off of.
Does anyone know what is wrong/What can I do? (She's still just as active as before, still happy to see me, still eating, and diet, location, etc is still the same.)