Welcome to Health care magic. 1.The possible cause one can think of after a trauma, small bruise increasing in size - infective aetiology with collection of pus within. 2.Then vascular abnormality also could be cause, finally lymph nodal enlargement due to local infection / inflammation. 3.Definitive diagnosis in this case can be done after examination - i would have send 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.Mean while 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.