Likely is is because her skin is itching her. There are several things that can cause this: Parasites like fleas, rashes from chemical exposure, or allergies. If you can't afford a trip to the vet then go to the pet store and get some anti-itch spray and use it according to the directions. Inspect her carefully and see if you find any fleas on her skin. If you do then wash her with anti-flea shampoo and put a flea color on her. If this doesn't work then call your local animal shelter and ask them if they can refer you to a low-cost vet.

 My cat did that once and my vet told me I was hair mites and they will just stop after a wile and the fur will just grow back.

 Maybe she has hot spots, and keep scratching it, i suggest you to go see a vet