There's a small pimple under right armpit yesterday. it is painful when i press it gently. now, i noticed another one under my collar bone. i dont see any pus on the head. what cause this?

Infected cyst: It sounds like infected sebaceous glands or cysts, or if under the skin and not visible on top of the skin, enlarged lymph nodes which could be infected. I would recommend warm compresses to the areas several times per day and if worsening, not improving, fever, please call your doctor for either an in person or telemedicine visit.