Extended my contract without my knowlegde

I bought a one year contract and paid it off with Planet fitness in Pretoria,Sterland with an option to renew it or stay monthly basis.I didn't renew the contract and I asummed that it will automatically be on monthly basis or whenever i go to the gym,i will pay for that month. On December and January i didn't pay because i was denied access to the gym,of which i didn't have a problem as i didn't pay for those months.But suprisingly they send me an sms saying that i owe them the December and January payments