I had the SAME issue with my twin daughters about 13mos ago. They had the arching backs, a ton of spitting up, ton of gas, screaming, etc., and I felt like I was giving them poison the way they were after drinking a bottle. My ped had NO idea what to do, he recommended gripe water, mylicon - but they didnt help. He also wanted me to try a ton of formulas before he'd prescribe medication, or recommend a gentler formula. I eventually went to a GI dr at Morristown Children's Hospital, they put my twins on Zantac, and also told me to switch to Nutramigen. All I can say is this was a blessing for them. Before this, I would run out at midnight and get different formulas, b.c it was just heartbreaking to see them like that. We tried a lot of formulas.
It seems like their poop was different in that they were constipated, not sure if from the Zantac or the formula, but I would have to give prune juice in their morning bottles every now and then. Is the zantac working since you started it? If there is still pain, I would highly recommend Nutramigen. Pricey - but found that Target has them cheaper. Hope this helps!! Good luck!
* Also, if you have flexible spending, they will cover this formula (along with dr note)