Unexplainable usage of Internet
The easiest (and cheapest) way is to check the data usage through Activity Monitor. Just type in the name in Spotlight to launch it, and navigate to the Network tab. You can sort by bytes sent/received, though I think you'll have to keep it running to get historical data by app (otherwise shows only total data sent/received). 

For a complete solution, where you would get prompts for all apps trying to access the internet, and the ability to create filters etc., you can check out Little Snitch 3 (https://www.obdev.at/products/littlesnitch/index.html).