How Can Menstrual Cycle And Ovulation Days Be Calculated?

My menstrual cycle changed from 28 days to 35 days suddenly.. it was almost a 28 days cycle since my marriage (2.5 yrs). But this month my period occured after 35 days.. That is my last period was on April 12 th and this month it was on 17 th.. I am preparing to become a mother , so this sudden change makes confusion on the calculation of ovualtion days.. Please help me..