What Causes Cotton Allergy?

I am in my sixties and about 5'10" since I went to vietnam the second time I have not been able to wear cotton on my skin, I get rash, itching that is extremely bad, and with in about 2 hours the smell is like that of cat urine, my solution has been to wear nylon or polyester, my problem is in the underwear, all I can find that has any comfort is Bali brand womens undies, I have done this for years, it seems no one has any explanation for this problem. I have tried creams, powders, lotions, and nothing has worked. Do you have any ideas?
Thank You