After keyhole surgery to remove ovarian cyst under general anasthetic i had almost projective vomiting for 4 hours, what could this have been due to?

Multifactorial: Certain factors raise the risk of postoperative nausea and vomiting. Some of the factors include gender, type of surgery, location of surgery, anesthetics used, post op pain and individual predisposition. Gyn and laparoscopic surgery, female gender are risk factors. I don't know what anesthetics were used but i imagine medications that can cause the side effects were in play.

Mild allergy: Histamine release can happen secondary to the anesthetic agent and in rare instances can happen. Does not mean you are allergic to the anesthetic but you are just sensitive and next time you may want to mention this to your anesthesiologist.