No, you are incorrect. The IMDMA has undergone significant changes since 2001. 750 ILCS 5/510 covers subsequent modifications. Many post-decree modification have been filed as a result because the percentiles have gone up.
Ergo, you will need to refile your 13.3.1 & 13.3.2's, get your subpoena's filed, and grab a good lawyer. I've seen support orders DOUBLE in circumstances analogous to yours - so I would urge you to contact the Chicago Bar Association and talk to an attorney to see what your options are.
Your question is a good one and it stems, in part, from an odd thing in IL law.
In one part of our child support law, it requires that child support payments be equal to the applicable percentage guideline. In another part of the same law, it requires that all child support be paid in fixed dollar amounts, and not in percentages. In another part, it allows for percentage amounts under certain circumstances.
However, if there are no unusual circumstances, then child support must be in a fixed dollar amounts which is geared to a percentage of base pay. Bonus, commission, and overtime income is always extra, or in addition to, base pay amounts.
Your question cannot be answered properly without knowing some other things, e.g., what it says in your judgment, whether your increase was due to a base pay increase or because of other reasons, etc. It is safe to say that you will likely be paying more if you continue to earn more, but your ex may not be able to collect back pay on the particular increase you mentioned.