From what I have read, women who have been on the pill for several months or even several years may experience a gradual thinning of their uterine lining. This is actually one of the ways in which the pill prevents pregnancy - it begins to make your uterine lining too thin to hold onto a fertilized egg IF you were ever to ovulate and conceive.
Also, most people say this is NOT at all a sign that your pills have not been working at doing what they're supposed to - mainly, preventing ovulation - but it may mean that your estrogen/progesterone levels are slightly out of whack and therefore might be having a harder time holding onto your lining before it's time to "let it go"!
I have been experiencing bleeding a few days before my inactive week on and off for the past year (I started BC about 16 months ago.) I'm going to my gynecologist and I am going to hopefully see if I can switch pills to something with a better estrogen/progesterone ratio for my body.
Good luck and good health to you!