Looks like you have a problem.
Most hospital admission forms include a provision for who is responsible for the billing. If you put your ex-husbands name and insurance information, they would probably be going after him (at least initially). However, you probably put your name down and signed the form.
Here is what will probably have to happen: Make sure that the hospital has the correct insurance information for your husbands provided policy (per the decree). Then, the uninsured portion should be first billed to your ex-husband. Any amounts that he doesn't pay (or all of it), the hospital will come to you for payment and you will have to pay to protect your credit. You will then have two scenarios:
1) You can get an attorney to file for recovery of your damages, and contempt of decree, in the original divorce court, and/or
2) You can sue your 'ex' to recover your damages. Assuming it is within the Small Claim court limitation, you can sue there and save your attorney fees.