dog doesn't want to play or walk?

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I need some help. I have a 2 year old Australian Shepherd that for a week now has not been himself. His symptoms are all over the place. A little history first though: a few days prior to his symptoms he had an allergic reaction (stung by something) and the Vet gave him a shot in the bum muscle of Benadryl and Prednisone. We noticed a few days later he just didn’t seem like himself. He didn’t want to play with his brother (our 9 month old Aussie) and all he wanted to do was lay around. We then started to notice a limp and didn’t want to jump up in the car. We thought maybe he hurt himself while running and playing outside earlier. But then the limp went away and now he just seems stiff…and then limps again..then stiff. I have given him Metacam but it hasn’t helped. He is eating, drinking and going to the bathroom as normal. Today he didn’t want to go for his walk…of course once he got outside and moving he seemed ok. I asked a third party if they noticed a limp and they said “no”. He is walking at a slower pace and seems almost depressed. I have been chatting with his Vet but it is hard to pin point what is going on. It has been a week now. I know that we will probably end up taking him in to the Vet again tomorrow or the next day but it is going be expensive to weed out what is going on…blood work, X-rays. Could it be the sting? could it be from the shots in the bum? Could they have hit something while giving the shot? If it was a leg injury wouldn’t I see improvement in him with the Metacam? If someone said their dog was acting tired, stiff and depressed…what would you think? I have been researching West Nile Virus, Lyme disease, Bone cancer….I am going crazy….and making myself sick ;(