After open gallbladder surgery, i've had really bad lower right side back pain and spasms. i mostly slept sitting up after. can that be the cause?

Maybe: Would need more information. How long ago was the surgery? In the first few weeks after major open abdominal surgery, there sometimes can be some back muscle pains due to abnormal posture while sitting or standing. Try changing positions, stand up more straight, walk more, use local heat on the muscles. See your surgeon to be sure that you are healing properly.

Hard to tell: Although, surgery predisposes you to back issues due to some forced position during it, it's hard to tell whether your back spasms are a result of your recent surgery or another problem. Make sure to check with your surgeon to rule out any potential complications of the surgery and if so, treat the spasms symptomatically - stretching, heat, massage, topical rubs etc.