Cellcept and Pregnancy

I've been on cellcept for three months. My doctor told me when I first starting taking the medication that i should not get pregnant while taking this medication. My husband and I had sex once during my ovulation and now my menstural is a week late! I am so scared. We really messed up! Also, I was taking 5mg of prednisone and my doctor took me completely off it about 2weeks ago. I don't know if my menstural is late because I stopped taking prednisone (idk just a theory, this is not the first time my period has been late because of prednisone) or if it's late because I'm actually pregnant! I am so dreading taking a pregnancy test. Does anyone know the effects that cellcept can have on your pregnancy? Keep in mind, that I've only been taking cellcept for a little over 3months