I had blunt force trauma to my upper arm, after two weeks i still have a large painful lump under the skin after two weeks. not visible. what is it?

If it is just: Under your skin in the subcutaneous tissue, probably a soft tissue hematoma. Apply heat and massage the area gently. If it persists beyond one month or so, it could be something more, so get medical evaluation.

Most likely blood: You most likely have a hematoma under the skin, this is a jello like collection of clotted blood which accumulates under the skin after trauma. It is usually just treated with pain meds and a heating pad. There could be other causes, but this is most likely, should resolve after 2 or 3 months, if not, see a doctor.