Hi,
You should avoid using a deodorant directly to your under arm skin, avoid shaving or waxing the area for some time. These could be the cause of your symptoms.
Try to avoid sweat accumulation at the sites and use a powder. Keep the area clean and wear lose, comfortable clothes.
Since when have you noticed the lesion?
This could be some insect bite, eczema or bacterial skin infection or dermatitis.
You could take oral antihistamine medications for the itching and apply calamine lotion at the site.
It would be best to consult a skin specialist as nothing can be said with surety without having a look at the lesions.
Let us know about what the doctor advises and post us if you have any other doubts.
Regards.
Could be hydradenitis (sometimes spelled Hidradenitis). If so, hot compresses might help but for me tetracycline and shots/drainage in a dermatologists office were the only things that have worked. I hope you don't have it. Good luck.