Bleeding 2 weeks after period, on pill for year and a half, missed 2 days. Negative pregnancy test. What could be wrong?

Bleeding 2 weeks after period, on pill for year and a half, missed 2 days.

You missed two days of the pill? It could be that was enough of a drop in the hormone to cause a period to start. Continue taking the pills as normal and make sure to use a barrier when sexually active. A condom for example.