It looks as though someone burned you on the upper arm with a small car cigarette lighter; roughly circular and with pin pricks where the vaccine needle was dipped and jabbed repeatedly in the epidermal layer of flesh at the apex of the outer shoulder muscle.
Several folks have mentioned other places to look if you don't find it on the upper arm. Mine is on the underside of the upper arm to avoid the unsightly scar problem. Other places mentioned were hip and buttocks.