What Causes Recurring Rashes On Armpits And Behind Knees?

this is a reocurring rash that began in armpits/ now inside elbows and behind knees. i am sure heat is a factor as my daughter is a softball pitcher. latest treatment is topical cream betamethasone dipropionate 0.05%. this relieves some of the symptons but rash does not complete go away. We need something treat rash, itching, burning and preventative suggestions for the future. she is a talented athlete so exposure to sun and heat will remain in her future.