What Causes Drainage Coming From Incision Before Getting Periods Post C-section?

Hi Dr. I had a c section 5years ago and I Have a drainage coming from my incicion every month when I am due for my period. I am feeling very embarrassed about it so I kept quiet, but it is smelling bad and my incicion area is then very tender red and swollen. When this happens I wipe the drainage liquid of and try to keep it dry but where I wipe it spreads. What is wrong with me and why do I keep getting it every month when I am due for my periods?