Why is my dog lactating?

I have an 11 year old Dauschund who recently started lactating. She has never done this before. She has not been in heat for quite a while (6+ months). She is presently being treated for canine arthritis and is taking Tramadol and Prednisone. Could it be a side effect of the medication? This has never happened before and was just in to see the vet last week, but I will take her in again if necessary.