Am I late or should i be due on next week?

I normally have a regular cycle of 28 days. Last month my cycle was only 21 days. I had unprotected sex on the 12th day of this cycle and if my cycle is now 21 days i would now be late for my period indicating i may be pregnant. My question is being a week early for your period a once off or will that be the new pattern to my cycle and therefor should i do a pregnancy test seen as i am 2 days late? Thank you in advance