What Causes Late Periods?

Hi Doctor, I am 43 and my menstruation cycles have shortened to 21-25 days from the previous 30 days. This happened for the past 2 years after i stopped the pill and use condoms instead. We have been faithfully using condoms and this month my period is late. My last one was on 15 March 2011 and anticipate this one on 10 April. About 10 -12 days ago i was so tired one day that i had to lie down to rest - could it be implantation? I am anxious as i do not want to be pregnant at this age. Jackie