What Causes Menstrual Cycles Only On Intake Of Birth Control Pills?

A friend of mine had a spinal tap last Thursday to drain fluid bc of a tumor on her optic nerve(cannot remember the name of the tumor-it s not cancerous). Any how she began her period the following night (which is very rare for her-she stopped birth control pills in January and hasn t had one since-and she typically won t have a cycle unless she s on bc). Not sure if there is a correlation? She also has a headache now. Can you help?