Hi I'm A 40 Year Old Caucasion Woman. I Have

Hi I'm a 40 year old caucasion Woman. I have had a skin reaction under my left armpit for about a week. Its pinkish, itches at times and after a shower it hurts. I have not put deodorant, or shaved under there for a few days without any effect. I have not changed laundry soap or body wash or deodorant. I am at a loss. Only place is under the left armpit. Please help! Thank you