Your employer just wants to make sure you are safe while working. Your doctor should have written you a release statement.
Very normal for a workplace to ask for a medical release if you’ve been having known medical issues. I work in HR and have had many pregnant employees with a plethora of job affecting symptoms. We always require a medical note from them that is with or without restrictions and an end date of those restrictions when they bring up concerns. It’s to make sure that the employee is safe to perform their job for themselves and others. If you are in the United States, they cannot fire you due to pregnancy, it’s illegal. But they can require medical documentation and if you refuse to do that and continue to miss it could be termination worthy. Your doctors playing dumb and needs to provide a release or give you restrictions.
Work release is a pretty common thing if you are having medical symptoms that are affecting work. It’s basically just a paper saying you are safe to be at work and any restrictions you may have
Do you work for Amazon?