I don't think your doctor thinks you're making it up. There are a lot of things out there that can cause pain that doctors just never figure out. It doesn't mean you're just "imagining" it. Don't worry about that.
How would you describe your pain? Constant, intermittent, burning, achey, sharp, dull, etc.? Do you notice a pattern with any particular foods?
If your gallbladder is working okay (which it sounds like it is), this could be a musculoskeletal problem. There's a lot going on in that part of the body other than the gallbladder. Does it feel worse after you eat, drink, sit, stand, etc.?
I find it's helpful to see patterns if you keep a "diary" of sorts of your symptoms. When the pain occurs, how bad it is, where exactly you feel it, etc. and then you can show that to your doc.