I'd be of the opinion that a positive result would be unlikely to show up just 9 days after sex.
I don't think that's enough time for the sperm to find the egg, fertilize it, have the fertilized egg travel all the way down to your uterus, implant and have your body produce a high enough level of pregnancy hormone to get a positive.
Also, given your period was due sept 18th, your most likely ovulation date would have been around sept 4th or thereabouts.
All indications are that it is not your husband's baby, although, the only way to confim who the father is with certainty is a paternity test