If the rash in your groin area is due to fat thighs, you might try to use a few different products to see exactly which one works for you. I know from experience that, sometimes, your body might not react as fast to one way of healing from a product as it would from another product is why I am going to list several products or homeopathy that you might try.
I, too, have suffered at one time from a rash in the groin area, from what I was thinking was because of fat thighs....let's hope it's from that, and if not, it may be jock itch.
Either way, I would try these products (in order they are in), and if none of those work, you might ought to see a physicians assistance.
Here they are:
1. Cornstarch--I like to use cornstarch (purchased at the grocery store/walmart/dollar general in baking section) since it takes out the moisture from that area and will help it dry on up. You might sprinkle some in your location of rash where it rubs in your clothing to help you throughout the day.
2. Gold Bond--it will help ease the itching and if it's jock itch (know this, women can get jock itch from guys that have jock itch...just in case you are a female), it will help to ease it and heal it.
3. Triple antibiotic cream--I would use this if the other two products didn't work because it may be more serious.
I hope this helps!