I do not know of a place specifically that will tell what is using data in the sense is it streaming music, uploading files, etc. but below are some thoughts on things you can do to monitor your overall data usage.
At this page, https://customer.xfinity.com/help-and-support/internet/data-usage-what-is-the-terabyte-data-usage-plan/ under the Available Tools heading is some information on how to check your data usage. Using these tools, in combination with tracking what you are doing on the internet for a given time period might help narrow down what is using the data.
Information on this page, https://customer.xfinity.com/help-and-support/internet/data-usage-will-i-be-alerted-data-usage will help you setup alert notifications so that you can be made aware ahead of time that you are reaching the data cap limit.
Hope this information helps you investigate your situation and find out what is going on.
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