Hello Mom,
It's only natural to be worried. 

From the picture and brief history, it seems your baby has a Yeast Diaper rash.

How does a baby get yeast diaper rash?
Everyone has harmless amounts of yeast (Candida) in and on their body. This fungus thrives in warm, wet places, like the mouth, bowels, skin, and vagina. Even if your child is a boy, the moist environment of a wet diaper can easily cause a yeast infection ‚especially if there's already an untreated diaper rash.

Babies taking antibiotics and breastfed babies whose mothers are on antibiotics are also more susceptible to yeast infections. That's because antibiotics kill the good bacteria in the body that keep yeast in check. Without these bacteria around, yeast can grow more abundantly.

If your child recently had oral thrush (a yeast infection of the mouth), he may end up with a yeast infection in his diaper area, too. Yeast passes through your child's digestive system when he eats and ends up in his stool, which eventually lands in his diaper right next to his warm, damp skin.

What's the treatment for yeast diaper rash?
Using vaseline, regular diaper barrier creams or ointments won't help.

What will work is:

• A topical anti-yeast or anti-fungal cream
• A mild corticosteroid cream
• Apply the cream two to three times a day, rash should clear in a few days
• you could use a barrier cream over the medication to keep the rash from getting worse.
• Change diaper frequently.
• Gently clean diaper area with clean water and pat dry, don't rub.
• Use alcohol free and fragrance free wipes.

Dr.Mims