Hi and welcome to HCM.
Thanks for posting your query here.
In a regular 28-30 day cycle the period of ovulation will be on around the 14th day. From the day of ovulation if the ovum is not fertilised it results in menstruation
on the 14th day- so total a 28 day cycle. First we need to calculate your ovulation day.
As you are getting an irregular period of 28-35 days your chances of ovulation could be from 14th day to 20th day. The period of ovulation is called as fertile period. You had sex on march 13th which is out of the fertile period. Hence chances of getting pregnancy
In a doubt case please check your urine sample for pregnancy on the first day of your missing period -april 6th. Because of irregular periods
you can wait upto 4 to 5 days and have a check again. Urine sample check can be done at home with pregcheck, etc which is a very simple test.
Hope this helps to you.