Hello hope the answer helps you.As you have a large sized nontender lump which hurts on flexion and extension.As sir you donont have local inflammatory signs and not running fever so it is unlikely to be abscess,boil or furuncle as having pus.It could be any lipomatous swelling as lipoma,or containing nerve fibre as neurofibroma till proven or ganglion like cystic lesion along synovial tendon sheath.Dull ache I guess is due extrinsic compression over nerves going in arm and foream bcoz of size.You dont worry if not associated with any systemic upset as Gastrointestinal tract or respiratory better consult general surgeon sir hope the answer lessen your anxiety