You still have a uterine lining. It's just very thin on the pill. The drop in hormones makes you shed this lining. In a normal period without the pill, you shed a thicker lining plus the egg you ovulated. On the pill there is no egg and there is a thin lining. This is why some forms of birth control like hormonal IUD and long term combo pills without break weeks eventually cause some people to stop having bleeding altogether.