If it has been going on for over a month I would get it x-rayed. or at least a second opinion.
I am guessing it is her front leg?

My rott had an on/off limp & x-rays showed her sesmoidal bones (the tiny tiny feet bones) were fractured. It is not common but not too awful rare either, mostly in large dogs. My vets & a specialist say that depending on the age of the fracture & the age of the dog they may heal themselves, or there is surgery but about a 50/50 cahnce it will do any good. Now this was about 4.5-5 years ago so they may have better options now. My dog was only 6 months at the time so we opted for meds. They worked for a while then it didn't bothere her for a while then it flared back up & knock on wood no problems for the last couple years.

But I was advised not to even walk her unless necissary. My personal opinion is that is a dog is limping and you do not know the cause I would not excersize him/her at all until it is determained.