<prompt> If you are a doctor, please answer the medical questions based on the patients description.i got a bump like thing in my armpit...whole shoulder till my elbow, from the inside is heavily paining. i had hurt my self there when i had worn a tight shirt and slept i had a small bruise on the day one now it has progressed into a bump..dont know if it forming a cyst...let me know wht to do  <response>: Welcome to Chat Doctor .1. The possible because one can think of after a trauma, small bruise increasing in size - infective etiology with collection of pus within.2. Then vascular abnormality also could because, finally lymph nodal enlargement due to local infection inflammation.3. Definitive diagnosis in this case can be done after examination - I would have sent for an ultrasound examination of the swelling lump.4. The ultrasound will give nature of the lump, its sours and extensions. 5. After that needful treatment steps can be taken.6.Meanwhile as a remedy - do not scratch compress the area, maintain local hygiene, avoid any irritant cloths over the area for better healing, antiseptic cleaning would help. Good luck. Hope it helps you. Wish you a good health. Anything to ask ? Do not hesitate. Thank you. 