Easy question. Macules are flat. Papules are raised above the surrounding level of the skin. Hives are itchy papules where each hives lasts less than 24 hours. Hives often last for days to weeks but an individual hive lasts less than a day, resolves without leaving a mark on the skin, and may be replaced with other hives.
Viral or allergic. Fleeting rashes are often allergy, heat, or viral. It is very hard to know without seeing it so I would suggest taking a picture of it and making an appointment with your pediatrician/physician. Taking a picture is very helpful since, like cars with funny noises that are gone when you see the mechanic, rashes can disappear when you see the doctor.
Noticed a recurrent macular rash on my glans penis after intercourse. No pustules, no drainage, causes pruritis. Dermatologist or urologist?
Either. This may be sensitivity to vagninits, feminine hygiene product. The use of a condom mmsy prevent rash. If there are no signs of infection topical Hydrocortisone and non sedating antihistamine to prefer rash.
Derm. See a dermatologist or your primary care physician first.