brake pedal seemed to start going further down on its own

Bought the car a few days ago. Car was driving and stopping fine. The next day I was driving slowly in traffic and tried to stop. The car slowly came to a stop. While at a stop the brake pedal seemed to start going further down on its own and the car started to slowly move. I had to press the brake down further and harder to get the car to stop moving. I have never experience this before and it has happened several times since with no warning. It sometimes does it when the car is on the highway at a speed of fifty five to sixty mph.I have to be extremely forceful to get the car to stop. I have had the brake fluid checked and the rotors. I was told that it was because i was short? Shortness does not cause a brake pedal to start slowly moving on its own. Help please.