What is making my broken foot swelling up?

Injury to the bone: The break in the bone it the final stage of an injury to the foot, there damages to the tissues of the foot which cause the swelling.

Many things: Some swelling is needed for healing. But increased swelling can be caused by walking too much, not elevating the foot, infection. If excess, you need to see your doctor.

Instability.: The swelling is your body's way of stabilizing the injured tissue for initial tissue repair. It is, in a sense, nature's cast. The more movement of the injury site, the more swelling will occur. Reduce the movement to reduce the swelling.