Gemoney Carecredit Paid in advance and now they're say I have to pay again!!!!! Internet

I pay $112 per month to GE/carecredit.  On 1st October I paid $112 (this was October's payment due). 

On 11th October I also paid for November and also for December - total of $224.  As far as I was concerned the next payment was due/is due January 2010.

Last week I received an e mail reminder telling me my payment for December was approaching.  I called today to ask why I was sent a reminder etc.

I spoke to a Indian/Pakistani who told me that the December payment is due tomorrow December 2nd.  I tried going through the dates and amounts with him, telling him that I've already paid for December.  He told me "you still have to pay the $112, which is due tomorrow.  You can't pay in October for the bill due in November". I repeated that I'd paid on 10/1 and 10/11 and these payments went through.

He agreed that the amounts have gone through my bank but still said that I have to pay December's instalment tomorrow.

I've told him that I won't be paying again for December and that the next payment they would receive would be January's.

Where do I go from here?  Who should I contact? I just know they're going to pile on charges  but  I'd done my budget, planned in advance, paid up to date and to be honest I dont have another $112!!

I'm absolutely furious!!