I had bad gastric pain after having prawns. does this mean im allergic to prawns?

Definite maybe: If you also had hives or difficulty breathing the answer would be yes. A stomachache alone could be due to something other than allergy. If other people ate the prawns & only you had symptoms an allergic reaction is more likely. I suggest seeing an allergist about this. Simple blood or skin testing & an open oral challenge (in that order) will give you the answer & the safest path to take.