Starting early 3rd month of BC

I'm 24 yrs old, and about 3 months ago, I started taking Ortho Tri-Cycline Lo birth control. For the 2nd month in a row since starting, I have started bleeding a couple days before the placebo pills started...is this normal?

Additionally, is this a sign that I started my first month NOT with my natural period? Am I more prone to pregnancy if my birth control timing is off from my natural period?

Finally, I recently got an ultra-sound to check for abnormalities due to extreme cramping, and they did not find anything. However, they did say I could potentially still have endometriosis. Could this cause the early bleeding?