What is causing my heavy data consumption?

That all depends on what you are doing on the iPhone. Any video streaming would very quickly eat up that tiny data cap. Even the heavy use of social media would quickly use that much data. The following article will instruct you in how to determine what is using your data:

https://support.apple.com/en-us/109323

iPhone->Settings->Cellular->Cellular Data->Show All and look for the culprit. You will also see that you can turn off cellular data usage for individual apps.

cwterry5 wrote:

What is causing my heavy data consumption? I have 50gb limit and I am over that in less than a week.

We couldn't possibly know what's using your data. But you can check yourself on your iPhone by opening Settings > Cellular > Cellular Data and you can then choose Show All to see what is using data on your phone.

Ive AT&T as a carrier so your results may differ. Please follow this path,

iPhone > Settings > Cellular > scroll down > please see screenshot