Hard lump on lower part of inner left calf muscle when flexing. not present on other leg. also odd numb/spasm feeling in same spot when walking stairs?

Need to be examined-: And investigated by an orthopod. This might be a muscle hernia or a bony lesion, or a ruptured tendon ( retracted and rolled), or a hematoma (collection of blood which has clotted). Difficult to say what exactly? See your orthopod. Good luck..

See doc: A hard lump should be evaluated by a doc especially if it hurts and is associated with nerve symptoms. An MRI may be necessary and a biopsy if it is not evident as to what it is.