i doubt you are pregnant but i would indeed take a test in about a week to be sure....its not unusual to have the odd period due to stress,diet or just feeling down in general but if you are still concerned i would pop in and see your doc to confirm all is well
good luck
You wouldn't be pregnant 2 days after having sex, so that's definitely not what caused this. Most likely your cycle was just off this month - starting early and being a little different than usual. It happens. You could also have STD, or an infection or cyst or something. If you're not up to date with any smears etc then I would suggest getting those done.