It sounds as if you have a contact rash. These rashes are most often caused by dyes and chemicals in clothing, cosmetics, soaps, detergents.
To relieve the itch use a cortisone cream, they are many available over the counter and can be purchased at a drugstore or grocery store.
You may also find that you may need to change your deodorant and laundry soap. Try also to wear loser fitting clothes so as not to irritate the rash further.
If the rash grows or persists you will have to see a dermatologist or your primary care physician as this could be a sign of more serious conditions.
Good luck and best wishes.