What causes your hair to fall out after surgery and what can I do to make it stop.?

Depends on chemo. Some forms of chemotherapy attack the systems in a cell that cause it to divide and grow. This is a good way to fight cancer but it also targets things that grow quickly like hair, the lining of your mouth and intestines. Some cancers have specific targets for which there are drugs - these do not attack your hair. Depending on the cancer there may or may not be a more selective type of chemo.
Stress. Stress from surgery can cause a temporary halt in hair growth cycle, resulting in hair falling out. This is generally temporary and you will likely regain the hair lost.