Pulling out small patch of hair on leg

I have been told that before going into season a female Jack Russell may sometimes pull out some of her hair on her legs.I have a female close to 8 months old, and she has a small patch of hair on one of her legs that she keeps pulling out. She has'nt had a injury there and she dose'nt have fleas. Also, how old are they when they go into season and how long do they stay in season ?
Thank You,