Is a prenuptial any good if signed after marriage?

Well, preliminarily, if you signed such an agreement the "month after we were married," it is NOT a premarital agreement, but a POST-nuptial agreement. Most States have enacted some version of the Uniform Premarital Agreements Act (UPAA), but State law governing marital agreements entered into AFTER marriage varies enormously from place to place. In some places they are valid, in some places they are limited as to subject matter or terms, and in some places they are not permitted, essentially, at all.

You should probably seek consultation with an appropriate family law specialist. I would suggest starting your search with the American Academy of Matrimonial Lawyers, which can be viewed at http://www.aaml.org. Good luck.