Talk to General Physician on Sudden Appearing Of Lump In Armpit

AOA! i am 35 days postpartum and had delivered my twins in 36th week of pregnancy via c-section due to sudden up and down in uric acid and bp... my babies are on mother feed and top feed both... my query is that from last night I felt pain in my left under arm and when I checked, I could feel a loose lump which is ‘painful’ but apparently there’s nothing on the skin. Can the doctors here tell what can be the possile reason for this and if it is something dangerous? The lump doesn’t feel very deep though but it hurts...