I had sex once during my o and once 3 days after my o, i am suppose to start in 3 days, i have really nausea and tired and crampy.could i be pregnant?

Yes it is possible: Wait for three days and if you miss your period do a in home pregnancy test and it will tell you if you are pregnant in future use protection against pregnany, like use of a condom or consult your doctor about other options like pills or Depo-Provera shots etc.

It is possible: It is possible to be pregnant as this is most fertile period to get pregnant but difficult to say but wait until 3 days if you miss period then you see your m.D and have him run pregnancy test or you could do it yourself at home.