For starters, naive in thinking notarization conveys any enforceable legality to your agreement. All it does is indicate that the two people who signed the paper were, actually, who they claimed to be. That's it.
Further, in not understanding that child support is never carved in stone. If she is unable to support your (joint) child? She has every right to file for support at any point in time, no matter what "agreement" you came to.
Further more, not bothering to do any research wrt support and how it all works.
Was paternity ever legally established? If not, Mom will need to have that done before she can have support ordered (be advised that you may not only be ordered to pay back to the filing date, but also further back than that). Of course, until paternity is established and you have an order for custody/parenting time, Mom is under no obligation to allow you to see the child. And no, that will not mitigate the support you owe.
And a further word of advice - make sure you wrap up for any future romantic encounters, whether the girl says she's using birth control or not - your swimmers are YOUR responsibility. You are in no position to have another child at this point in time.