Which other than pregnancy can cause you to miss your period on the pill?

OCP and period: It may be normal for you to have very light or no bleeding on birth control pills. The amount of bleeding relates to how thick is the lining that develops on the pill. This is influenced by the type of medications in the pill, your weight, how active you are, other medications you are taking, and whether you miss any pills. When you miss or have light periods, you could be pregnant as well.

Being on the pill : Some people do not have periods or have very light periods when on the pill. If your pregnancy test is normal then this is usually ok.