Common symptoms of an armpit rash include itching, small bumps on the skin, redness or darkening of the skin, flakiness, pain and increased odor. More severe armpit rashes can cause dryness, cracking, pain and blistering of the skin, leading to fever in some cases. Several factors can cause your armpits to break out in a rash. Some common causes include improper shaving methods, excessive use of chemical-based products, excessive sweating, regularly wearing tight and synthetic clothes, exposure to a hot climate, friction and poor personal hygiene.