My 10 year old active, healthy and happy dog has sudden symptoms of pain?

My 10 year old active, healthy and happy dog has sudden symptoms of pain. He is slightly hunching back and cannot seem to get comfy. It is now Friday, 4 days ago he was trembling for a couple of hours. That stopped that afternoon. He vomited 2 times that night BUT the whole week he is eating well, drinking water as usual, has regular urine and slightly less but still good BMs. At the park he runs and plays with his buddies BUT indoors he is whimpering and in obvious pain. Seems to be abdomen. On Thursday I took him to the vet. He had an xray and found nothing (no tumor thank goodness!). Vet says he MIGHT have a bacterial infection or inflamed intestines. Gave us four meds. Anti fungal, anti diarrheal (although he does not have loose stools), anti biotic..but no pain killers. Any ideas? He is a healthy active happy boy otherwise. He does have arthritis but this seems to be IN his tummy. He does not eat trash but does eat white snow in the park. Help.