Why do dialysis patients have bumps on their arms?

Fistula dilations: We use fistula (shunt) which is a vein that is connected to an artery. The artery pushes high flow blood into the vein to make it large and suitable for dialysis use. That vein is called arterialized vein. The nurses put the needles in that vein to perform the dialysis treatment. The place left from the needle is weakened & after frequent use gives out (dilates) that appears like bumps under skin.

Aneurysm : You may be referring to areas in the dialysis graft or fistula which can form called aneurysms or pseudoaneurysms. The fistula can become dilated and this will look like a lump or bump.