What Causes Heavy Bleeding When The Contraceptive Pills Are Switched?

I am on the combined pill and i am bleeding even though i am not on my week break. I have just switched from another combined pill. Before changing from cilest i back to backed 2 months, i took 2 pills of the second month and then switched straight onto the first pill of my loestren sheet. Is this normal? It is heavy breakthrough bleeding. I am 19 and have been on the pill for over a year.