Discharge at 4 years old??

My daughter only turned 4 last week and the over the past week I've noticed a little bit of discharge in her underwear, maybe 3 times. The first time I noticed it I checked her and she was very red and sore so I started to apply a cream for yeast infection. This has cleared up the redness and shes no longer sore but I noticed today she still has slight yellow discharge in her underwear again. I'm calling a doctor in the morning but what could this possibly be?