Although gallbladder disease is widely discussed by docs as a product of Three F syndrome (Female, Forty, and Fat), weight loss can trigger it, too. Gallstones are basically little calcified wads of cholesterol. If your body is transporting to much cholesterol in either direction it can start forming stones. In the case of weight loss, you are "reversing" the normal flow of cholesterol in your body - repackaging it from cells that no longer need it.
Maintain a moderately lowfat diet until your surgery, and especially avoid sudden big influxes of fat. When your stomach receives fat, it signals the gallbladder to contract to squoosh out bile that emulsifies it. The more fat, the bigger the squoosh, and the more it hurts.