they refuse to cancel my expired contract

I had a one year contract with the gym that expired in February so I was on month to month.Last week Thursday I went to the gym to cancel my contract I filled in the forms then on Friday I got a call from Jackie saying I will be liable for 3 more months because I didn't cancel within the minimum period, am so angry at them,I just want this to be over,the contract period has ended why am I still liable.