Hi and welcome to Healthcaremagic. Thank you for your query. It may be umbilical hernia most probably and this is benign diseases which may be serious if become incarcerated. At this point you need to rest, and avoid physical activity till pain subside. LAter you can consult surgeon about surgery. Other causes are less common and may be some other benign subcutaneous lesion such as lipoma, fibroma or atheroma and if there are no skin changes such as redness, pus discharge or pain and fever then you should not be worried since it can be easily surgically removed under local anesthesia. I hope I have answered you query. If you have any further questions you can contact us in every time. Kindly regards. Wish you a good health.