Imac using lots of data or maybe verizon scam?

i have a iMac a few years old never had a problem with data usage until i switched to verizon sorry so long in advance

first i have a broadband router from verizon with 28 gigs of data available per month

7 days after starting the service (dec 15) received an alert that i had used 50% of my allowed data i called and they confirmed that maybe my computer had updated and used the data i turned the computer off and went out of town the next day no devices hooked to the wifi and the computer turned off i used another 10 gigs of data while out of town.

i returned on jan 2 on jan 7 i get another email that i had used 50% of my data i called and was told in so many words sorry about your luck i at that time unplugged and put the router back in the box on jan 8 i went out of town. on jan 10 while still out of town i get 4 emails 2 min apart 1. you have used 70% of your data 2. you have used 80% of your data 3. you have used 90% of your data 4. you have went over your slotted monthly allowance yes all while my router is not even hooked up or not even power to it .

i called and told them to cancel my service on jan 13

today i plugged in router just to see if they had cancelled my service of corse they had not so i imideately unplugged the router and called them. the guy tells me that in the last week i have used 2 gigs of data remember it was completely turned of and had no power to it

my question is is there something i can put on my computer to monitor data and also what could be pulling data either on my mac or other?