What Causes Irregular Periods After Having Sex?

HELLO DOCTOR I JUST WANTED TO KNOW THAT MY GIRLFRIEND IS HAVING IRREGULAR PERIODS.ACTUALLY DOC WE HAD SOME RELATIONSHIP ON 17-04-2010.On that day i just got only a inch of my Penis in her vagina and There was a couple of drops of blood. so we just stopped and there was no semen out from my penis....she got her periods on 16th may n that was for around 8 days..after that she is having irregular periods.so doc i just want to know is ma gf pregnant if yes then what v shud do to cure it...