If the last period was shorter /lesser than ur period then it cud be due to implantation of egg which happens with 20 to 30% preg woman.Check for pregnancy.In case ur last period was normal in flow and length than there could be some other reason probably illness or stress.
how long is your cycle, you cannot get pregnant if you have sex and then your period comes shortly afterward, are you sure it was a period, not just spotting etc, was it a normal one for you? Also stress can delay a period as well