Absolutely! Get a lawyer, right away.
The lawyer will likely file for an "adjustment", the laws vary from state to state. They aren't that unfriendly to fathers in any State that you will pay support for a child when you have physical custody. You can't file for an adjustment without a lawyer.
The money you spend on a lawyer will be well worth the money you save. Make sure you get a real lawyer, and that you pay for his services. I spent $100 or so for an initial consultation which lasted an hour. By paying, I knew that the lawyer would give me real information rather than a sales pitch, and when you have the information you can make the decision about how to go forward.
It maybe you just need to file a motion and won't even need to go to court. But you won't know until you talk to a lawyer.
Get a lawyer right away.