It will become obvious soon enough.
I know that sometimes it freaks you out and you begin to make up possibilities. There have been several occasions where men have had children after vasectomies. They are not 100 percent guarenteed, and yet it is horrible when yuo are that 1 percent b/c it makes your spouse think you have been unfaithful.
Tell your husband, things will be hectic for a while but you should be able to with medical counsel explain to him that these things do happen and you have been a faithful and loving wife.
Good luck