The first thing you should do is close your eyes and breathe deeply for about ten seconds.
Now, make a copy of the money order, along with its endorsement, and send it to your landlord, along with a letter saying that, if she does not dismiss the action, you will sue her for malicious prosecution. If you do not get a favorable response, you must then file an answer denying the allegation and go to trial.
Your lease probably provides for attorney's fees to the prevailing party.
After giving your landlord proof of payment, if they fail to dismiss the suit, be sure you show up to present your evidence. Check the deadlines - if you can counterclaim, you don't want to miss the deadline.