I had unprotected intercourse with my boyfriend around 5/29 5am, later that day i started to get red bumps between my buttocks. thought of heat rash?

Heat rash v friction: Heat rash is like, as is a friction dermatitis rash. Either way monitor and if it lasts more than a few days or becomes painful, see your PCP

The key factors: are likely moisture and bacteria, often associated with intimate contact, irritating the hair follicles called folliculitis. A more worrisome red bump might be herpes, but I don't think you would notice the symptoms the same day of exposure. Dry skin/eczema/psoriasis also possibilities, but would be coincidence, not from sex. Try: gentle cleansing, benzoyl peroxide. If persists, need Doc eval.