Hi Jim,
Welcome and thanks for posting. We can help address your concerns. First you want to isolate your computer and run a malware scan. This way we can ensure there isn't a virus that's using your connection without your knowledge.
We can't tell what is using your data, but you may want to look into using a third-party data monitoring program to help you pinpoint the programs on your PC that use the most data. Community members have suggested and used GlassWire, so feel free to ask them for tips on using it:
https://www.glasswire.comThanks,
Liz
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