Given the data you provided (skipped one birth control pill one week ago, had unprotected sexual intercourse the next day, pulling out method used, now started bleeding), it cannot be determined for sure what is happening.
If no sperm has entered the vagina, it is unlikely that you are pregnant. Of course, there is really a small possibility that you became pregnant with pre-ejaculatory fluid. In this case, the bleeding might be implantation bleeding (which is usually light) or it might be a miscarriage because you kept taking the birth control pills as recommended (and birth control pills are not supposed to be used in pregnancy).
It is more likely that the bleeding is related to hormonal fluctuations caused by skipping one birth control pill. The suppression it provides ceased for one day and the hormonal production was probably messed up. This might lead to bleeding.
You might want to visit a gynecologist for a physical examination. Hormonal laboratory tests might confirm the hormonal imbalance. An ultrasound scan is good enough to confirm or to rule out even the smallest chances for pregnancy.
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