got charged even after DSL connection cancelled

I had O2 DSL internet without contract. I cancelled it to move to other internet provider. My new provider took care of cancelling O2 DSL internet. I even got email from O2 saying the last date of internet connection. Now O2 internet is stopped and new provider's connection is started. But I still got the bill for next month when I am not using O2 internet. What should I do?