Sensitive painful hard lump on my left leg just below my knee on the left side of my calf... Any ideas of what it could be?

Can be anything! Hard painful lumps and bumps need to be evaluated by your PCP, or dermatologist for further evaluation. Depending if there's an additional red ring around it, or if it's nodular, or if it has any other associate findings will determine what it could be. So a physical examination needs to be done for definitive diagnosis. .