my daughter had the same thing but on her arm i had no idea how it even got there it was a big black bruise and had a huge lump in it i took her to the doctor and the doctor told me it was somthing like a blood clot but nothing serious it would start to go away as the bruse faids it did within a month and a half but if it doesnt go away then i think maybe you should see a doctor it could be alot worst.
but here is somthing that helped know me alittle to know about bruises:
Most basic bruises are spread out and do not present any remarkable symptoms – that is, anything beyond the expected amount of pain and the telltale skin discoloration. Severe bruises may, from time to time, result in an uncomfortable, hard lump under the skin after a bad bruise. This is known as a hematoma.
As anyone who has suffered from them knows, hematomas may feel sponge-like, rubbery, hard or lumpy (or some combination of these characteristics). Despite how they feel, hematomas are not typically cause for concern – unless they occur on the head following a head injury (it's important to note that a head hematoma may not occur for several weeks following the head injury, so be mindful of its potential to appear). In this case, it's crucial to seek medical attention immediately.
As mentioned, hematomas usually only occur with more severe bruises. They occur when blood collects and pools beneath the skin after an injury. When this excess fluid accumulates, it creates the hard swelling known as the hematoma.