I have red spots less than 24 hours after sex. allergy or std, what is this and treatment?

Unclear: If the sexual practice is the traditional type the penis may have gotten some exposure to pregnancy preventative such as vaginal foam or a spermicide inside a condom. If it only occurs with exposure in this setting it is probably some chemical irritatant. It the partner is going through menopause , it might be some irritation from a lubricant. See your doctor. STD unlikely with these symptoms.