I had open cholecystectomy thursday 17th, last night I suffered from what felt very much like a gallbladder attack. How is this possible?

Retained stone. It is possible that a stone may have slipped into the common bile duct and is causing obstruction that was missed at the time of surgery. It could also be due to spasm at the end of the common bile duct and will go away spontaneously. Either way contact you surgeon today.