Comcast XFinity Over charge me Plymouth Meeting Pennsylvania

I was always a month behind on my bill and would get a notice to pay by a certain date to avoid shutoff and I would pay by that date. In October of 2014 they told me I needed to pay $400 to keep services on. I paid it. They then said I was still passed due $140.

i just gave them $265 on January 2015 and they say I still have a balance due of $286 ?? Where are they coming up with these numbers. That would be a total of $551 in 3 months and I'm only supposed to be paying $126 a month.

When  I question them they twist the numbers and debate me. I feel like they keep adding in an extra month. Also when I make a payment with a representative they refuse to give me a confirmation number. Strange ??