According to the data you provided, you might be experiencing hidradenitis. Hidradenitis is a condition in which a bacteria stays trapped in a blocked sweat gland, thus causing superficial skin inflammation. There are different hypothesis about the primary cause (clogging of the ducts or bacterial presence), so the exact cause is still unknown. Extended use of antiperspirants, deodorants or depilatories (hair removal products) might aggravate the condition.
You should avoid use of antiperspirants or other skin irritant chemicals including depilatories. Retinoids to open the gland ducts and corticosteroids to decrease the inflammation are used together with antibiotics if infection has occurred. In the most persistent cases surgery might be needed.