If you are concerned and not believing your vet about Deramaxx, here is a link with some information. I believe he knows your pup well enough and has seen her lab results in full and is more than likely correct in saying it was not the medication. If you are not happy with his plan, again I would suggest an internal med. specialist, preferably board certified.
Anyway...here is the link:
http://www.fda.gov/downloads/AnimalV.../UCM050385.pdf