Jack russel depressed? Hurt? other?

I have a 4 year old female non-spayed Jack Russel. She has always been very active and hyper. Lately she has been slow and "depressed" like but is still eating well and her nose is wet. At first I thought it was her knee hurting ( she hobbles sometimes) but she does not seem to be favoring any particular leg. She seems sad and it looks as if their is a glaze in her eyes. Does anyone know what may be causing this?