Bloated Sick with bloat.

It was all fine for the first few days, the guinea pigs seemed normal happy, friendly and bubbly little animals. A week had past and one of my guinea pigs (Mocha) had started making noises like she was in pain and she was grinding her teeth. It was 6'o clock and there were no vets open. She ended up making these noises all night. The next morning we immediately took her to the closest vet, but the vet was apparently "late" so we had to wait an hour just to get in. She had no idea what she was doing, she didn't even know what the animal was! She though she was a hamster -.-"! Then she made us pay $75 in a vet bill when she did nothing! We ended up finding a vet that specialized in rabbits and guinea pigs (An hours drive away). The vet gave her an X-ray and gave her some medicine and told us to come back in 2 weeks to check up on her. Mocha improved and no longer made the noises and her stomach went back to normal. The vet ended up figuring out that the pellets we bought from a pet store must have done it to her stomach and given her gas because nothing else explained it. After a few weeks and regular check ups with the vets the vet decided to allow Mocha to eat pellets again that were from the vet and we new were safe. Then everything started over again, a few days later she made the noises and we went back to the medication. We are going back to the vets tomorrow. The vet also said she could have a stomach problem. I was wondering If anyone has had any similar experiences with their guinea pigs or knows of any stomach problem she could have.