Without seeing what this looks like, it could be a number of things. Are the clumps of hair just sitting on her back like shedding and she simply can't reach or are they like spots from an irritation? Another thing I thought of is my mom's cat started pulling out her fur because she was stressed, could that be what's happening? Nothing has changed in the environment lately, has it? Sorry I can't be much help, but hope you can figure it out! If she's bright eyed and acting fine, I wouldn't worry too much about the vet unless it does look like some kind of skin condition.