What is the statute of limitation for medical billing in New Jersey?

I was treated at an ER in NJ almost 2 years ago and I paid all bills associated with the visit within th efirst few weeks after treatment. Two years later I received another bill for $350. When I called the hospital they said that the other bills were for the EKG, Xray, etc. and that the individual MD doesn't take my insurance. I find it odd that they waited 2 years to submit this bill and after I already paid over $500 for the visit I question it. Is there some statute regarding the length of time after a visit that a new bill can be sent out? Many thanks.