What causes red spots to just randomly develop on breast, under arm and upper leg?

Red spots: Fungus is first thing that comes to mind: either tinea or candida. Fungi like warm, dark, moist areas. Tinea/ringworm can come from pets or contaminated objects (gym mats, for example). Otc Lamisil (terbinafine) &/or powdered spray like tinactin/cruex/desenex r usually enough to clear infection, use for at least 1 week after spots disappear since deeper skin layers may still be involved. See doc if not better.