A painful lump in the armpit can be caused by swollen lymph nodes, a skin infection, blocked sweat glands, boils, cysts, shaving-related irritation, or inflammation around hair roots. Some lumps improve slowly even after starting antibiotics because swelling and inflammation may take more time to settle completely. If the lump is still painful after 6 days of Linezolid, the infection may not have fully cleared yet, or there may be a pus collection inside the swelling. Warm compresses, keeping the area clean and dry, and avoiding squeezing or pressing the lump may help reduce irritation. In some cases, ultrasound, blood tests, or drainage may be needed if the swelling remains large, painful, or persistent despite treatment. Repeated lumps in the armpit can also happen with sweat gland inflammation or chronic skin conditions. Rapid increase in size, fever, pus discharge, weight loss, hard fixed swelling, or persistent lumps lasting for weeks may need more detailed examination to rule out deeper infection or other causes.