hello. most common cause of lumps under the skin is lipoma. but lipoma is usually painless. you should get yourself checked out by a general surgeon. he'll examine you and tell you what type of lump it is. he may ask you to get certain tests like ultrasound, FNAC, biopsy etc to be done. suggest you to see your doctor as soon as possible when the following things happen with a lump: 1. increase in size 2. becomes painful 3. getting hard 4. grows back after its been removed. all the best.