It does sound like a yeast infection.
Yeast infections just happen and are not an indication of lack of cleanliness.....they are everywhere, and if your girlfriend had one, then so do you. Go to the drug store and look in the men's health aisle for a product to treat Jock Itch. If you cannot find one, look in the women's hygiene aisle for a product to treat vaginal yeast infections. You will find Canesten, Vagisil, or Monostat. All will work.
Apply the cream as per the directions on the tube. If it is a yeast infection, you will notice immediate results.
Continue to apply the cream as per instructions on the package and do not stop when the symptoms disappear,....else it will come back with a vengeance and you will need a doctor the second time.