I was served by a Collection Agency and reached a stipulated agreement but...?

I would get the settlement agreement in writing before paying anything. Are you making a lump sum payment? Or is this some kind of payment plan? If it's a payment plan, they probably will want you to sign a consent to the judgment. If they have already filed suit, they will want assurance that if you don't comply with the payment plan, they don't have to start over with the lawsuit.