If you are having gall bladder problems, not much of anything will help. Btw, I had a bunch of tests done and they came back clear. Several years later I had the same test over again (hydascan) and it showed up a bad gall bladder. Mind you, I knew it was my gall bladder for 5 years. Tests are not the be-all, end-all because they certainly failed me.
You haven't said anything in particular that sounds like gall bladder vs. IBS, but here were my symptoms: horrible, horrible pain (couldn't stand up straight) in the upper abdomen. A lot of people hurt exclusively on their right side, but mine was across the diaphram. A lot of people are tender too on their right side around and up under the rib cage, but I didn't have that until the very end. Sometimes after eating I would hurt into my shoulder and down into my right arm. This especially happened after drinking coke, although caffeine and carbonation aren't triggers for everyone. I would wake up in the middle of the night in terrible pain and would pace the floor until I had a bowel movement which was some matter and a lot of yellow water--which burns like fire. A lot of people with bad gall bladders vomit predominately or vomit and have diarrhea alternately, but I only vomited with mine once; I was diarrhea predominant. When you are having a gall bladder attack you cannot lay down! I think this is unverisally true for everyone with a bad gall bladder. It makes you hurt worse and makes you sicker. Also, after I would have my middle-of-the-night bm, I would gradually start to feel some better. Never well enough to lay back down immediately, but the pain would start to subside. Then I would go back to sleep sitting up in the bed. I would take terrible chills during and especially immediately after the episode. When it was being unruly but not in the middle of a bad flare, mosts foods would make me nauseous just to eat them. And sometimes I would suddenly feel like I was overly full--sick you feel so full--when I hadn't even finished my food.
If most of that sounds familiar, then it's probably your gall bladder. Having a colonoscopy will gain you nothing. If you haven't had a hydascan, then you're missing a large part of the equation. While ultrasounds will show large stones, smaller stones and sludge can sometimes be missed. And then there was my gall bladder. I didn't have stones at all. It just didn't function properly. The hydascan tests gall bladder function. Make sure you have it done while you are in the middle of having problems. My gall bladder would work for a while, then it wouldn't. When it was working, it showed up on the tests as being fine. That's why they kept missing it.
I don't think Immodium will harm you, regardless of what undiagnosed problems you may have. People with IMS, Crohn's and UC all take it. If a doctor hasn't told you not to take it, and especially if they've recommended it, then I would take it. Still, I find that they constipate me the next day, so I have to keep it to 1/2 a pill at a time. Only take what you need to stop going to the bathroom, and allow 30-45 minutes for it to kick in. Most of the time when I was sick with my gall bladder, though, I used Pepto Bismol pills. They stopped my diarrhea without constipating me. They don't contain aspirin, but you can't take them if you are allergic to aspirin.
You can also try calcium supplements. I have had very good success with Caltrate 600 with Vitamin D with two pills a day. Some people take three, some only one. Depends on what it takes to stop you up. If it's your gall bladder, though, it won't help until you have it out. Calcium will not effect excess bile. However, Immodium and Pepto Bismol will help with any sort of diarrhea, whether caused by water, bile or parasites, etc.
I have read that soulable fiber isn't beneficial to people with diarrhea and may make it worse. You need insoluable fiber to provide bulk. The other kind just disolves in water and won't help if you have excess water--you need the insoluable kind to help bind up the water.
One last thing: dehydrated banana chips. They'll constipate you quicker than anything else I've found. My theory is they get in your guts and rehydrate themselves and absorb all the excess water. 1/3-1/4 of a supermarket bag is enough to harden my stools for days. I will caution you, though, if you suspect, after reading this, that you have a bad gall bladder, do not eat the banana chips and be sparing with the Immodium. Because if your gall bladder is bad, it will put too much bile into your digestive tract at once. And that stuff will want to come out regardless. If you've made yourself a bit constipated by eating banana chips or taking too much Immodium, you'll just get really, really sick and will eventually pass the hard stool followed by fairly explosive diarrhea. If you don't think you are having a gall bladder problem, then by all means, eat some banana chips. I ate some this past week and hardened up my stools enough that I was able to eat all sorts of fatty, greasy foods this weekend and not have any problems.