Implantation bleeding is a drop of two of blood.
Women spot. It happens for dozens of reasons or not reason. Implantation bleeding is one of the LEAST common reasons for it, and it's really silly to leap to the conclusion that you are pregnant because you are spotting.
And not clear what you mean by 'I had unprotected sex.' You are ON BIRTH CONTROL. By definition, when you are ON BIRTH CONTROL, sex is PROTECTED. That's what birth control IS!
While a missed or very, very light period while on the pill CAN mean you are pregnant, if you took all your pills correctly, the risk would be very, very low. More likely you are just having a very light bleed or missing one. It happens.
You could test to set your mind at ease, but I very much doubt you are pregnant.