Abby didn't throw a fit while being cinched either, she just pinned her ears. She only got really mad when I touched more towards her flank.
Normally they scope them. My vet is really big into acupuncture, so she put her little laser on Abby's "stomach spots" and apparently they were really hot (I know next to nothing about acupuncture). She said she wouldn't be that surprised if the cause was ulcers just because Abby's a fairly anxious horse and she had been moved from her home of 4 years (she's only 7, so over half her life).
But she was definitely grumpy for the month or so during diagnosis and treatment. The loping problem might also because when they have ulcers, anything I think over a trot (Not 100%, but that's what I think it was) splashes the acid around in their gut and causes pain. Abby threw a fit at a lope because of it.