Brief Answer: Hi and welcome to HCM Detailed Answer: If there was hit then this is probably subcutaneous hematoma and it should subside within few days. Probably several small blood vessels broke and this is nothing serious. If you dont remember hitting then it can still be sponatneous hematoma and this is commonly seen especially in girls. Rarely it can indicate some blood clotting disorders,but if there is another such lesions on your body,you should check your protrombine time and clottign factors, trombocytes. Thirdly,if there is pain around it andvisible vein,then it can indicate superficial trhombophlebitis. In this case you should rest with your arm,take nonsteroid painkillers, apply cold compresses and vitamin K cream or heparin gel. This can be done also for hematoma that I ve mentioned above. In every case, you should not worry at this point. It will pass soon if I am right. At your age,some more serious diagnoses are rarity. Wish you good health. Feel free to ask. Regards

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