It is hard to say what the hard lump under the wound is without a physical exam. Also, the dimensions of the hard lump might have been helpful. You might be experiencing a post-operational hematoma deep in the skin (not visible) that is absorbed and replaced with connective tissue. If the hard lump does not disappear within a few more weeks, or the skin around it becomes red, hot, and irritated you might want to have a follow-up with the surgeon that removed the tumor.