My grandmother has a small bruise on her upper-inner arm and it has a fingernail sized lump underneath, she only noticed it a few days ago, cancerous?

No tumor: lump appearing under a bruise frequently results from hematoma consolidating to produce a palpable lesion. Tumors take time to develop. In rare instances a tumor appears at site of bruise but from a pre-existing cancer, removed and appeared under control. This lesion should be watched to assure no further growth and if not resolved while clinically stable, should have excisional bx