What are side effects of Rimadyl?

Jackson, 5 mo old boxer, is on second round of Rimadyl and Clavamox for reocurring UTI. This time he's been on meds. for 4 days. Today I noticed he isn't eating or drinking very much. Yesterday, I noticed a rash on his belly side mainly down around his back legs, belly side. He is also having like fast leg movements, back legs, and shivering motion I noticed today on front legs. He's also sleeping ALOT!! Is this from Rimadyl? Can I give Jackson motrin or something I have here for pain? Thinking I'm not going to give him anymore Rimadyl from what all I've read about it. Vet told me there were no side effects.