Cancelled Service in March and just received a bill for 3 months of service

I cancelled my service a few days before my yearly promo contract ended on March 8th and before the start of the next billing cycle, but never received a confirmation call like the online service said I would, then moved to a new house shortly after that. On March 19th I received an email from an external source through xfinity that said they couldn't reach me, but I had never received a call. I called the direct number March 22nd and spoke to someone who cancelled my plan. So my account should not have had any due balance and that was all good to go.

Fast forward to July and I just received forwarded mail from my old service address, that I haven't lived at now for about 4 months, late last week that says I have a past due bill for $121. I owned and used my own router/modem, took it with me when I moved and kept my wifi password protected, so its not like someone could've kept using my services this whole time after I moved. I'm trying to get this sorted out now and would like my account fully cancelled/closed out.