There are myriad causes of hives (urticaria). One category of these physical and one physical type is Exercise, not rarely associated with a specific allergy to foods or food allergy. In its mild form it may be no more than a nuisance but in the more severe form, this can be associated with swelling below the surface of the skin or, quite importantly swelling of the vocal chords and surrounding structures, or even progress to anaphylaxis.
The most important message is that this is a potentially serious condition, one for which you should seek consultation with an Allergist/Immunologist, preferably at a major medical center. Do not delay in your efforts to have this condition evaluated and treated and do not engage in exercise until this is accomplished. Pre-treatment, prior to exercise, can in some cases prevent what you have experienced.
Good luck
Thank you for your reply! I'll make an appointment asap.