You certainly have a lot of good advice here! I agree with most of the posters that first, go NOW to the doctor and make sure it's a pregnancy, and that it's not ectopic.
Then, depending on who you are, what you believe, and what you can handle, decide if you will abort, carry the baby to term and place for adoption, or have your sixth child. I don't believe "things happen for a reason." I think "sh-t happens," and you just have to deal with it, one way or the other. The issue is whether you can deal with a sixth child.
And finally, yes, it's time for your husband to have a vasectomy!