Get to your gp as soon as possible. I expect they will allergy test at the local hospital.
My ds came up in hives the first time he had butter when he was 7mo. The gp prescribed us piriton and he was allergy tested and got the all clear, butter and all dairy is now fine. It was very odd. Although he isn't allowed to have nuts until he's 3 in the autumn and the gp will let us try it out there just in case he's allergic.