Known endo, previous fused bowel, previous partial bowel ob's, last surgery on endo 12/15, now having severe pain jolt through lower abdo when sitting?

?complications? Sorry to learn of your pain. You may be having pain from adhesions(scar tissue from prior surgery)or other complications of endometriosis. Best to check with your Gyn specialist. If the pain is severe seek immediate medical help. Get well.
My first thought... ...is adhesions. Adhesions are fibrous bands that form between tissues and organs, often as a result of injury during surgery. These bands can prevent muscle and other tissues and organs from moving freely, sometimes causing organs to become twisted or pulled from their normal positions. Your pain requires urgent evaluation, probably with ultrasound or CT. I suggest an Emergency Room visit.