I'm worried that I'm pregnant.?

I had on the 29th of January and got my period on the 1st of February (at least I'm pretty sure it was my period... the days were regular.) It is now the 6th of March and I haven't gotten my period. I've had mononucleosis and bronchitis terribly within this past month and I've been on antibiotics. I've also been fairly stressed. I use natural family planning as a form of contraceptive.