Normal scans and labs are reassuring. As part of the lab workup, were amylase and lipase drawn? Those pertain to your pancreas. The GI doc can order those. Although an ultrasound can miss a common bile duct stone that can irritate the pancreas as well, the HIDA scan is pretty good at looking there.
Although clostridium difficile is something that can be present even when stool antigen tests are negative, if biopsies were done during the colonoscopy you'd expect that they could see evidence then that was more definitive.
It's worth discussing this more with your Gastroenterologist, since there's pain. It's hard to say whether on not gallbladder removal will cure more than your pain, it may not get to the bottom of your diarrhea.
Did you have blood tests for Amylase and Lipase? These are pancreatic enzymes. Just curios if they were elevated.
I should add I am in my late 20s and overweight.