The cat is an indoor cat she is about 4 yrs old she started licking he 2 back legs so much that they are bald she doesn't throw anything up and she is eating normal she has also been checked for fleas see nothing should this be a concern?

We have 3 ourselves... the oldest has undergone some weird behavior including what you described.. we were concerned and brought her for an exam.. nothing specific was found and she stopped on her own after a few weeks.. vet said maybe introducing 2 new younger cats spooked her some... try to discourage her from paying too much attention to the area... there is a liquid you can rub on that doesn't taste real nice that also might stop her/him... good luck