This HotSpot has a set amount of data it can use therefore the issue is with the device.
I know this won't be much help either but I'll mention it anyway. Some do not realize how much stuff they have running in the background that use the internet to update various programs ...apps on smartphones or if using a laptop or tablet, updating virus programs and other things that update automatically when they detect an internet connection.
An example, one guy I know didn't realize that his laptop was doing a Windows update automatically and it took a tremendous amount of data as his Windows hadn't been updated in awhile. He didn't realize that he had gone through that much data and had to buy more service cards than he expected. Many virus programs automatically update themselves or load definitions when an internet connection is detected in the same manner.
And, as you know, many websites have stuff running on the pages themselves that even if nothing is being done by you have videos or flash programs running, etc.
What I do when using a personal hotspot (I have several) is to not leave them running when not being actively used. Shut the hotspot off entirely when you are finished doing whatever it is you are doing on the internet. Do not walk away with a browser open and the connection running.
This also helps to prevent anyone able to hack your password to the hotspot from using the connection while it is running. When the hotspot is shut down, it is of course not using any data at all whether it is you unknowingly using data or someone else who has hacked your password.
I believe you are limited to 6GB per month of data with the cheapest service card. No, you won't be able to download/stream movies and/or videos with this limitation. You first need to determine how many emails, what kind of gaming you may wish to pursue and other data-eating requirements, such a downloading photos, videos, You Tube interactions, etc.