FYI-if you want to go less chemicals then desitin, Zinc oxide it the ingredient that makes it so protective and you can buy that on its own at any pharmacy (its GREAT for burns also!)
Well I think they have the what to do explained really well. So I'm going to throw some stuff out there for why its there and how to prevent it....
1. Has your diet changed in the last couple of days? (more acidy foods or spicy that would make his urine/stool more acidic/change in enzymes)
2. Did you turn the heat on/up at your house or start putting him in warmer clothes?
3. Where is the rash exactly at? Is it avoiding certain areas?
4. What the rash look like?
5. I know you said you change the diaper every time its dirty--how often is that (like in hours/min). ALWAYS make sure bottom is dry before putting on the diaper--so air dry or pat dry but just make sure no more whip wetness is on him.
The post c/s drugs you probably look and ANY TIME you take antibiotics or most meds- can cause you to kill off your "good bacteria" in your system..you may need some probiotics and they have baby safe natural probiotics also!
The reason I ask is because depending on what kind of rash it is depends on how you treat it...it its a heat induced rash you don't want to put a think cream that will force the body to hold in more heat--it would make it worse then

Or if it was a yeast issue then you need an anti-fungal and desitin will not work.
List of the different types of diaper rash-once you have which one it is you should be able to prevent it.
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Friction rash. This is the most common form of diaper rash, and affects almost all babies at some time. It develops when sensitive baby skin is rubbed by wet diapers. This results in a red, shiny rash on exposed areas.
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Irritant rash. Irritation can be caused by the diaper or by the acid in urine and bowel movements. This rash appears red in the area where the diaper has rubbed and is normally not seen in the folds of the skin.
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Allergic rash. This may occur in combination with an irritant rash or by itself. It is also more common on exposed areas. The skin looks like poison oak.
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Intertrigo. Moist heat causes this common type of diaper rash that occurs deep in the skin folds. The involved skin looks thin, as if it has lost several layers.
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Seborrhea rash. A salmon-colored, greasy rash with yellowish scales characterizes this diaper rash that is also worse in the skin folds.
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Psoriasis. Not a diaper rash per se but can start on bottom and spread. This stubborn rash doesn't necessarily look distinctive. Other signs of psoriasis usually accompany the diaper rash, though, such as pitting of the nails or dark red areas with sharp borders and fine silvery scales on the trunk, face, or scalp.
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Candidal infection (yeast infection). Is beefy red with sharp raised borders and white scales. Small satellite lesions surround the main rash. (this one you would want to put Desitin
over the Lotrimin, and if it didn't clear up after 3days of this treatment see a doctor. Also giving baby or taking more probiotics can help prevent this!