Hello,
This bump is due to formation of a boil. Boils are caused by infection of the hair follicle by staph aureus. Bacterial skin infections can be spread by shared cosmetics or washcloths, close human contact, or by contact with pus from a boil or carbuncle.
Apply warm compresses and topical antibiotics like Neosporin on the boil and keep the area clean. Take some over the counter pain reliever and consult a doctor for examination and antibiotics prescription. Severe cases may need incision and drainage. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I have recently had pneumonia for 6 weeks, and was on a potent antibiotic for 10 days then 5 more days as it didn't clear up. I still have a cough and now have developed the red, very itchy, large bumps (like boils) under both arms. I had yeast infection after taken the antibiotics (which I really hate to take) so I got some suppositories and cream for that. Cleared up in two days then the bumps came. I really believe that the antibiotics have upset the chemical structure of my body and this is one of the results. Antibiotics won't help as these are not boils but I believe, are some sort of yeast, allergic reaction to the past medication. I have had them for a week and they are less itchy and the original ones are less sore but still red and a bump. I do think that a prescription cortizone cream might work and I will see my doctor this coming week. I have been putting on a compress of Witch Hazel to cool down the itch and heat of the bumps.