Thanks for the link. I looked at all of them and none of them seem applicable, but obviously I can't diagnose myself. The discharge involves multiple ducts (if that's what they are) on both sides, so I think that rules out some of the conditions listed like mammary duct ectasia?
The discharge started around the same time that I started getting migraines, when I was 12. I didn't start taking my migraine medications until 4 years ago.
Is it possible that nothing is wrong and this is just a quirk?