Can u still get pregnant after a vasectomy?

About a year ago, b4 I even knew my boyfriend, he had a vasectomy bc some girl had gotten pregnant. It was not by him, but it made him realize he didn't want to take any chance. . He has 2 kids, I have one. He never went back to the dr 4 them 2 check his sperm. Has not had with anyone since her, until me. . We've been together a month. I am (as of today) 5 days late of my menstrual cycle. I'm not always *normal* but I took the past 6 mths and divided it by 6, and got the average of what day I should start. 29.8, rounded to 30 day.. Today is day 35.. He is the only one I've had with lately, and I'm concerned if I could poss. b pregnant regardless of his vasectomy, or if its just stress causing me 2 b late. I *thought* I was gonna start the other day bc of the *feelin* I had but day 5 later, still no sign. . Please help!?!?