Having gallbladder attack but no gallstones noted last November on CT scan of abdomen. What else can cause an attack?

Gallstones. Sometimes gallstones are of lower density, similar to bile, and if they are small, they may be difficult if not impossible to visualize on CT scan. So you could still have gallstones. Ultrasound would be the beast way to visualize them. Functional gallbladder disease/gallbladder dyskinesia, without stones, could also mimic biliary colic.