Autoimmune or Infectious
The fact that the dog has allergies indicates she is already predisposed to excessive 
immune response to various stimuli. In addition, the limping did not resolve with the 
administration of anti-inflammatory medicine ( we do not know the dose or medicine 
given however so this lack of response is subjective).  But if we put these two together 
there is the possibility of immune complexes being deposited in the the joint area ( which 
would probably not respond to antiinflammatory meds).  Autoimmune diseases can be 
wax and wane as well.  There is also the possibility of localized septic arthritis as well. 
There is not any documentation of puncture trauma or procedures done on the joint but 
since the problem started while ago it could be an ongoing problem. If the joint was 
injured previously ( one year ago) it may be more susceptible to infection if itŐs normal 
barriers are broken down.