I think it's extremely unlikely that you've conceived after eight years without problems with your husband's sterility. However, I know that, on rare occasions, the man's vas deferens can grow back together.
I'll be totally honest - vasectomies rarely fail, and I think it would be borderline impossible for your husband's to fail after so many years. Is it possible that you have developed some sort of reproductive ailment? In the case of ovarian cysts or pcos, you may feel pregnancy symptoms such as nausea and vomiting, irregular periods, and abdominal pain. Do take a test, and no matter what the result is, it would be good to seek a doctor's opinion to rule out any serious problems that are not related to pregnancy.