Sudden increase in data usage and sales pitch for unlimited service

Starting in June we started receiving over-use charges and have sought to understand where these are originating.  I checked my modem (not XFinity) and do not see high usage by any device in the logs and the total usage comes nowhere near the stated amount shown by XFinity logging.  Called technical support and they were unable/unwilling to provide any detailed information on the usage.  In researching this, I have come across mention of a technical glitch that was occurring - is this continuing? Customer service was less than helpful - defaulting to upselling services at a significant cost increase.  I have been a long time customer, but this recent episode and timing against upselling new services leave a bad impression of XFinity overall.  I have subsequently called back and upgraded to unlimited to avoid the massive overcharges, but have received NO realistic explanation of how XFinity is seeing excessive data usage that does not appear in my modem.  If you are going to Bill for over-use, please provide evidence supporting the charges.