Account errors

I payed off my contract from JustGym under Planet fitness last year around July, and last week Thursday I was called by a debt recovery agent, telling me that planet fitness has handed me over. First of all, I was never alerted that I still owe anything from the gym, no messages or calls where made to tell me that. Secondly, how do I still owe when my whole contract was paid off, and I still paid charges that even the consultants at the gym couldn't even figure out why I was being charged. Planet fitness must know that I will not pay that money, because I don't know what it is for, and my contract is fully paid off. So they must go make their investigations, and sort it out themselves. I will not pay it.