Hi! thanks for sharing your recent health problem with us! As per your description, the lump in your hand is solid, large, painful, inflamed and possibly within a tendon or joint that s why it is not felt or visible when you make a fist. Therefore I would like to consider nothing but an infective pathology in your case! I think there is nothing to worry about this but you definitely need surgical attention as it could be kind of ganglion, bursa, inclusion cyst, parasitic cyst, tumors or even tuberculosis also! Therefore, you must see a surgeon for a clinical examination, few tests if necessary, rest of the joint and some medicines or surgery if needed! regards,