What Causes Light Vaginal Discharges?

I have an embarassing problem! Iv noticed a light brown discharge that appeared a few days ago in my underwear. I have also noticed it on the toilet paper, but it isnt heavy enough to need a pad. I took no notice of it the first, but it keeps returning every couple of days. I am on day 17 of my cycle, iv been on ovranette birth control pill, and iv started taking Eltroxin for sub clinical hypothyroidism (about a month and a half ago). Please help i cant afford a gp visit!