I have bad news and good news for you. The bad news is: yes, your overtime counts as part of your income available to pay child support. The good news is, the Pennsylvania child support guidelines take into account the reality that overtime has its ups and downs.
Since your overtime may fluctuate over time whereas child support is a constant obligation from month to month, your income will be calculated based on at least a six-month average. If your overtime is partly of a seasonal nature, you will want to be prepared to show at least a year's worth of income.
If you think you might be facing a child support lawsuit, now is a good time to organize your tax and income records. Meanwhile, if you want to learn more about how child support is calculated in Pennsylvania, I have provided a link below to an article about that in my professional blog.