Hi M.. My son was on antibiotics for ear infections numerous times and got several yeast infections as a result. It sounds just like what you're describing. He often got them on his genitals but not his bottom, too.
The best treatment I found was:
1) Keep the diaper area dry - change baby every 2 hours.
2) Wipe with water and TP or cotton balls only - wipes will irritate the skin further.
3) At night use a larger size diaper to absorb more wetness.
4) Apply Lotrimin cream (Clotrimazole)twice a day for about a week. Don't buy the extra strength stuff for athlete's foot - it's too strong for delicate baby skin.
5) Use a light dusting of baby powder to help keep the area dry. However, do NOT use cornstarch as I made the mistake of doing on night. I was horrified when I found the rash had gotten much worse overnight! My mom, a nurse, explained that the yeast actually feeds on the cornstarch which allows it to grow like crazy.
Yeast infections can become horribly painful in a very short period of time, so you don't want to wait too long to start treatment. If it doesn't start to clear up in a couple of days, I'd head to the doctor as something else may be wrong.
Hope you get it under control!
Smiles,
L.