Help please

I had surgery and prior to the surgery I organized a payment plan. $1222 down (50%) then 12 monthly payments to pay the rest. Perfect. I have paid 2 of the payments. They claim they only received one, despite my records...and they now say I owe an additional $4800. This was under insurance at my previous job. I called and asked for an itemized bill. How can they quadruple my bill after the payment agreement is made....while I'm still getting emails about the 12 month payment plan at the same price. Can anyone suggest anything going forward. I'm not a citizen but a permanent resident and feel like I'm getting screwed.