If your employer believed you were pregnant (even though you weren't), you may have a case for pregnancy/gender discrimination. If you informed your employer that you were not pregnant, and you were simply fired because other employees decided to make false statements about you, you're probably out of luck. Unfortunately, as an at will employee, you can be fired for any reason so long as it is not discriminatory or retaliatory.
DISCLAIMER: This response is for informational purposes only and does not form an attorney-client relationship. It represents only the educated opinion of the author. It should not be relied upon as legal advice. The response is based on the limited facts provided, and might change given additional or different facts.