How is this possible ....

Been using cox internet for at least 15 years never had this kind of issue before . My billing cycle ends on 21st . On 23rd I get a text I went over 50% over my data so im like what's going on I open it and it says on 22nd I used 60gb and then on 23rd I used 815gb . So of course im like must be some kind of mistake I call and they transfer me to tech 2nd lever and he writes a ticket. Next day I see another 620gb of usage . So again I call and the 2nd lever tech is like we checked and everything is normal . So of course im like it can't be normal my average usage is 30-40gb . Long story short she was extremely rude and making it seem like it's my modem's fault which never had issues before and it's only 3 months old. Basically I got forced into unlimited plan. Next few days my usage was 60, 58 , 78 . Funny thing is I live alone and watch YouTube and Netflix on 720p . What is my option ? Could it be my modem or they just wanted extra money out of me for unlimited .