I work in womens health, I deal with birth control and abortion.
A week and a half after sex, you would not even actually be pregnant yet.
Pregnancy is not instant, it is a 2 week long PROCESS.
Most likely it is what is called a FLUKE, sometimes these things happen to women that have used the pill for 2-3 years.
The pill doe snot cause a miscarriage, and bleeding is not exactly a sign of pregnancy.
Most likely it is just a fluke, and we do not know why this happens.
Keep taking the pills.
Antibiotics and certain herbal supplements are known to cause spotting in women on the pill, this is because they lower the hormones in the blood stream.
If you took the pill later than usual, that too could be a cause of bleeding.