Rash in both elbow joints and armpits, appeared 10 days ago. Also red itchy spots on lower legs and penis/scrotum. Been working out a lot and humid.?

Need to examine. It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. It would be prudent to see your doctor as lesions around joints and genitals are unlikely to be due to hot and humid environment alone.