What Causes Lack Of Period While On Contraceptive Pills?

I am taking micronor contraceptive pill and as i had two pph's after the birth of my child in November i have been put on ferrous sulphate tablets twice a day. I was having bleeds every two/three weeks since the pph's and my last bleed was on 1st Feb two weeks wouldve been 15th but nothing has come yet, 3rd week will be tuesday coming. But could the iron tabs be weakening my pill as i had unprotected sex 6days after my last bleed now i'm worried i could be pregnant.... Please help!!