Is it common to get hives after having sex?

Not common: It is not common, but for an individual with chronic urticaria it can be. If it happens once, would chalk it up to random event. If it is recurrent, I would suspect cholinergic urticaria, or maybe contact sensitivity to rubber or seminal plasma. You can try an over the counter antihistamine (be sure to read the warnings), but let your doctor know about it if it persists.

Hives post sex: Not common, but happens. Perhaps you are allergic to something associated with sex e.g. condoms, lubrication gels, vaginal douche, perfumes etc. Or possibly something you consume before sex . Keep a journal of everything associated with sex and the hives, and perhaps you will find the culprit.