What Causes Vaginal Bleeding Wile On Birth Control Pills?

I am on a birth control where I have a period every three months. I had a period Nov. 16 that lasted 3-4 days. Then around the Dec. 29 I started passing blood clots for about 4 days. Then on the January 8 I started bleeding again I went to my doctor concerned and was put on hormones. She said the bleeding will get lighter till it stops, takes a couple days. I have been on the hormones on top of my birth control for 7 days now and the bleeding is getting worse today with more blood clots. I have been bleeding a total of 15 days. What could be causing this? And should I see a different doctor?