Oh no! You mustn't lose confidence in yourself! I guess everyone has small lapses of concentration from time to time (I didn't see a red traffic light the other week and half my car was over the line before I managed to stop), and you don't even know if this one was down to you. Maybe you just can't remember because you were scared and your mind has blocked it out somehow. There are plenty of drivers out there who perform illegal or dangerous manoevers when they get impatient with other vehicles.