I'm sorry, we aren't doctors. The red line could be irritation from your skin folding over - if the area stays damp/sweaty a lot; however, since I'm not a doctor, I can't tell you for sure and would hate to tell you something that isn't correct. I would advise you to get your doctor to look at it and prescribe the proper med.
I don't believe for one minute that this is in your head.
I can't believe that a doctor had the audacity to laugh at you - what horrible bedside manner!!
I wonder if you have tried some type of powder or corn starch on the area to help absorb moisture, since it would seem that an area where skin folds over might sweat a lot. I don't have a lot of folds, but I do have a problem on my inner thighs and underarms sometimes. I put a bit of baby powder on to keep my skin dry and that helps keep it from chafing. There are a variety of medicated powders on the market that might help; or plain old corn starch.
You might also try a medicated diaper rash cream, but to me it would make more sense to keep the area dry.
These are a couple of suggestions; however, once again, please bear in mind that I'm not a doctor, so you might want to see a dermatologist.
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