What is the possibilities of becoming pregnant if my boyfriend had a vasectomy 6 months ago?

I have irregular periods and sumtimes im a week early or a few days to a week late. Me and my boyfriend has had unprotected about a week after my period. i have 2 kids and when i got pregnant with them it was always exactly 2 weeks after my period but this time i had unprotected a week after i started. I havent been late yet but im curious if there is a chance that i can become pregnant?!!