It's not possible to tell what a bleeding was caused by just by examining the timing and nature of the bleeding, but it does give you some hints.
In general, what you need to know about an implantation bleeding is that:
- It happens about a week after ovulation
- Only about a third of newly pregnant women have an implantation bleeding
- It consists of very light spotting, usually dark brown or red, not bright red
- It usually doesn't last longer than two days
Pregnancy symptoms don't tend to show up for at least a week post fertilization, if not later, either. If you took the birth control pill as directed, you are unlikely to be pregnant.