What Causes Delay In The Menstruation Cycle?

I have missed my period for 3 weeks. This is the first time in my entire life that has happened and I am turning 40 tomorrow. My husband had chemo treatments and a retro-parataneal disection surgery due to his cancer and we were told we could not get pregnant. Could I get pregnant or could something else be causing my missed period?

I am 5'8 275lbs and am otherwise healthy.