Sometimes when i'm doing something physical i get really hot, itchy, and red splotches all over my upper body. what could cause this?

Do these types of blotches go away away you come down to a relaxed state. Could be caused by intense sweating. Perhaps you can apply some powder to area before your physical activity.

Your symptoms are suggesting heat induced urticaria or cholinergic urticaria, essentially an allergic type of skin reaction to a sudden increase in temperature. Would suggest seeing your primary care doc to discuss and having a plan to alleviate such symptoms when likely to occur.