Have they even given you a hida scan to test your gall bladder? I'd do that before I let them put me on antibiotics for anything, especially since you have some of the symptoms of a bad GB. I try to avoid antibiotics like the plague, myself, because, as you found out, they can really mess up your guts. But sometimes, when you do have a bacteria problem, they're a necessary evil because they will help you in the end. Still, there is a breath test that's available to test for too much/wrong kind of gut bacteria. I'd definitely want that before I let them put me on antibiotics. If you have a bacteria problem, then the antibiotics will work. If you have a GB problem, though, they'll probably just compound your problem. I'd make sure they had tested for everything else under the sun before taking a stab in the dark with the antibiotics (and I'd definitely try them if there was nothing else that was left to try; Sarita's taken some on the off chance they'd help and they did).
Any gut problems that invovle diarrhea can cause weight loss. I lost weight with my bad gall bladder, then gained it back. A number of people on here have lost weight because of D. Most seem to have a hard time gaining it back, but whether you gain it back or not seems to be dependent on whether or not you ever get a break from it. My GB attacks came and went, so I was able to regain weight when I wasn't sick to make up for the weight that I lost when I was sick.