She's saying I owe back child support, I've been paying cash

Long story short, we had an informal arrangement and I've been giving her cash every month for the last year. Now her new boyfriend probably told her to formalize it and she's claiming I owe thousands in back support? I have some text messages and stuff but not official payment records. This is about to get ugly and I'm honestly scared. Do I need to lawyer up immediately or can we work this out first?