Has it happened to any of you???

my boyfriend had a vasectomy about 6 years ago. he is the only one i have every been with. and now i am having a lot of pregnancy symptoms. i haven't taken a test yet, but i have missed my period, and as i said i have a LOT of pregnancy symptoms. i was just wondering how likely it is that i could get pregnant, and has this every happend to any of you, or someone you know?