Is a bump over a fracture (a few weeks after occurrence) a sign that the bone is starting to heal?

Many possibilities . Make sure you are being followed by an orthopedist to monitor the fracture alignment. The bump could be swelling, new bone formation, or even displacement of the fracture so you want to confirm that you are healing in the correct alignment with an x-ray.
Fracture callus. Healing bone is called fracture callus and if near the skin surface (top of foot, back of hand) this callus can be felt. Most likely it is just about healed, if not healed. The fact that you can feel the hardness probably means the swelling is going down. All of this needs to be accompanied that you are improving. Best of luck.