Whats wrong with my dog?

Ok so my dog is a golden retriever male about 6 years old. For a few days now he has been weak, lethargic, when he is normally very energetic. He is showing no interest in fetch, or or toys. he has a slight limp in his left back leg, so we took him to the vet. He had Lime Disease before so they didn't test him because it would show positive. The vet didn't know what was wrong, so he gave him pain killers for a sprain and antibiotics for lime. I saw a pack of silica gel on the ground, but he didnt eat it. There are signs of him licking the gel pack though. Plz help tell me what is going on!!!!