itchy pubic area when wearing underwear?!

I been having this itch around my pubic area, every time I am using underwear the itch is there and if I decide no to wear underwear (100% cotton) the itch goes away in about a day, and if I try to use underwear again the itch comes back after a couple of days and does not leave until I stop using underwear again, could I be allergic to cotton, if so why it does not manifest when I use a cotton t-shirt? does anyone know what might be the causes of this? thanks in advance...