Child support is the right of the child, not the spouse. Therefore, the spouse cannot waive child support obligations. Child support is calculated by taking into consideration two main factors: time sharing and income.
If you stopped making child support payments during the time you were together again, it's not the end of the world.
Theoretically, if both parents earn the same amount of money, then the time sharing is what would tip the scales of the support. Let's assume for this example that both you and your ex make the same amount of money. If the previous time sharing agreement was that the child spent 75% of the time with the other spouse and 25% of the time with you, you would need to pay X amount of child support to assist with the time that the child was with the other parent.
Now, if you were both to share the child equally, only the difference in income would be a factor because the idea would be that you are each footing the bill for the child when he/she is with you.
Let's take this a step further. If you both reunite and are living together, it is understood that you would be contributing to the care of the child equally.
Therefore, if you can prove that you were paying for things for the child during the time that you were reunited and living in the same home, then any back pay in support that you would owe (if any) would be the equivalent of what your payment was supposed to be minus what your contribution was during that time.
So if your child support payment is, for example, $300 per month and you can prove you spent $230 per month in caring for the child while you were reunited, then you would owe back pay of $70 per month. If you can prove you spent exactly what the payments were or if you spent more, then you would not owe any back pay.
I hope this has answered your question.