Hello... I have a large, round, blue, hard lump in the back of my leg.... it wasn't there earlier today and I didn't even notice it until I bumped it, and now it hurts a little and some when I walk.... is it a possible blood clot? I need some opinions... Thank You!

I'd get it checked asap. It may be a clot, it may be an infected cyst or it could some sort of aneurysm.