Maybe we should call in Dr. House
I hate not having a diagnoses but I guess doctors need to rule out the basics first.
Have you had a any tests done? STD test plus that swap test?
Go back to your doctor if he doesn't know what it is, then ask for a referral to a doctor that will.
If it's a skin problem I'm guessing a dermatologist?
Also if you are short of cash there's a lot of free walk in sexual health clinics, considering how many people they see a day they might actually know what is going on.