The bottom line in my eyes here is that the OP's employer did make an exception for another pregnant employee (and actually made an exception for the OP for a time if I read correctly). As others have posted, employers do have the right to deny you work if you're not able to perform your full duties. But once they've made an exception for anyone, they must make it for everyone. That's where they made their mistake. It would be discrimination plain and simple to make accommodations for one employee and not make the same accommodations for another. And she's also got them on the fact that they have no written policies on this issue. If employees were not made aware of a policy, and nothing was put in writing, then it cannot be enforced.
OP, definitely don't let this one go. They do not have the right to terminate you in this case. Under normal circumstances, if they had a written policy that you agreed to by signing, and if they had never made an exception for another employee, then they may be within their rights. But you've got reason to fight this.
Oh, and by the way, Congrats on the baby boy!!