If it were me I would stop the Metacam immediately. There has been a lot of reports about this type of medication, mainly around dogs bleeding internally. My vet is wary of it, but I know there are a lot of vets out there who prescribe it readily without warning of the side effects. There are plenty of other products out there for arthritis etc without using it ie: Cosequin which you can get from the vets or cheaper still online from America, or glucosamine. I only heard last week about a dog who was prescribed metacam, and is now in liver failure. Try not to worry too much about the anaesthetic as vets operate on dogs much older, and take precautions, sometimes using one generally used on greyhounds. The panting could be because she is in pain. A stiff hind leg doesnt necessarily mean she has arthritis though, it could be because of the lump. If you are not happy with the vet, find another one.