Red Rash In Armpits, Yellowish Pus With Foul Smell, Hurts To Lower Arm. Cure?

I am a girl in her teens and a few weeks ago I noticed a red rash under both of my arms that looks almost wrinkly. They started out dry, but my mother said it was like diaper rash when I was younger and the doctor she asked said to put athletes foot cream on it, so she said she did and my diaper rash went away. But after I put athletes foot cream under my arms, the rash under my right arm went away, but my left armpit started oozing this yellow-ish puss, that does not smell good at all, and it hurts to put my arm down.