Hi,
This could be an allergic reaction to some personal care product that you may have started using recently under the armpit?
Contact dermatitis is an inflammation of the skin caused by direct contact with an irritating substance.
The allergic type of contact dermatitis is caused by exposure to a material to which the person has become hypersensitive or allergic.
Cosmetics, Fragrances, perfumes and other chemicals and substances are common trigger factors.
The symptoms are itching (pruritus) of the skin in exposed areas and/or skin redness or inflammation in the exposed area.
Topical corticosteroid medications may be used to reduce inflammation, if present. However if itching is the only symptom then all you need to take is an anti-histamine like Cetrizine or Loratidine to provide relief. You can also apply Calamine lotion or a menthol-based cream on the affected area to soothe the itchy sensation.
ref:http://www.myonlinewellness.com/topic/adam1000869
It sounds like your allergic to your deo