I don't mean to scare you, but I am a 33-year-old female with an 11-year-old little brother... and he was a post-vasectomy baby from my father who never got his vasectomy checked! We are thrilled and love him (obviously), but it was challenging health-wise for my mom who was 42 at the time (she ended up having cysts on her ovaries during the pregnancy and had to have surgery to remove her them while 12 weeks pregnant... her water broke almost resulting in a miscarriage, but the sac resealed... she was on bedrest the remainder of her pregnancy). He was conceived about 10 months after the vasectomy.
I also do medical transcription and one of the doctors we do work for is an urologist. The biggest thing he stresses in his post vasectomy reports is the need for followup!
You might as well just take a pregnancy test because if it is negative, you will not have the stress of your worry affecting your body, which along with the hormones from nursing, can also affect your period.
Having said all that, if you end up pregnant, let me just point out that my brother has been such a blessing, even though he was a surprise and the pregnancy was a very hard one! I have never met a woman who found herself unexpectedly pregnant regretful of that pregnancy after she sees her baby! It sounds like you have a great husband, wonderful kids, and with their support can handle anything!
Good luck!
Jen