The child support is one issue and the "roommate" is another.  If you truly did not know about the pre-divorce child support (that should have been spelled out in a Temporary Agreement or at the very least, the Final Decree) that can be one defense against being held accountable for a large portion - meaning you may be able to get the amount reduced, assuming that you've been paying fairly regularly or the amount is beyond your income. 

Regarding the "roommate", that would be something you might want to bring up in a counterclaim.

At any rate, it sounds like you could use counsel, especially if she has an attorney.