L.U.
California is an "at-will" state. Which means they can fire you for any reason as long as it doesn't violate federal law.
I don't think it's fair, but I don't think you have a case. Also, I am not a lawyer, so maybe I am wrong!
You really only worked for them for a year and a half and then 5 months of maternity leave *which is REALLY long in the US*
Sorry....this sucks...but I think you just need to look for a new job and not jump on the sue happy truck.