You could be required to pay it from as early as the time the petition for modification was filed.
If you found this answer helpful, let me know by clicking the "Mark as Helpful" button at the bottom of this answer. It’s easy and appreciated.
the law is that the modification can go retroactive to the filing of the petition for mofication - and is supposed to if the need and ability to pay existed at that point. So it may very well go prior to your mediation agreement if it wasn't addressed in the Agreement. But the Court often uses its discretion, and perhaps if you are lucky you can get the ex to agree to the increase from the hearing date forward. May want to disuss it with the ex before the trial; perhaps agree to some sort of compromise on the start date. Or if neither of you hd attorneys perhaps it won't be brought up. Another thing to look at is in the original petition did your ex ask for the increase in support retroactive to the filing of her petition, if not, there is caselaw that says that you can only get what you ask for so if she did not ask for back child support it should be prospective only. Diffficult to make these arguments if you do not have an attorney though.
Family law rules differ significantly in every state, especially procedural rules. Always seek the advice of an attorney in the state where your divorce was filed.