Yeast infection on my dog?

Kay

Hello,

I got home from work today and my boyfriend showed this to me on our puppy. It seems more red than usual and he has been more itchy lately (he has also been have a couple fleas on him here and there but I think they are dying from his Frontline). Could this be a skin or yeast infection on him or possibly an allergy to some flea bites? Luckily he has a vet appointment tomorrow so I'll bring it up then too.