My Dad shares my 4gb dataplan with a Jetpack. His data usage for July went to 6gb from a normal 1-2gb. It turns out his laptop was infected with malware. Everytime he connected, videos and things were running in the background. I downloaded and ran Malwarebytes. It found and cleaned like 20 things. The typical antivirus doesn't always prevent malware. Anyone running Windows needs good antivirus AND malware protection.
Luckily, when my data got close to the 4gb limit, Verizon sent me an email and I temporarily upped the plan to 10gig until I could get over to see what was going on.