This issue is an ATT service/account related problem.
As it states, you've used 5.9gb, but your data plan must be paying for only 5 gigabytes of data a month.
Ever site you visit, email you download, file you download, etc uses up data as part of your plan. If you are streaming music or video, that uses up a lot of data very quickly as well.
ATT has a provision on most plans that will allow you to exceed your data plan, rather than shut of completely so you aren't cut off entirely. However once you reach the cap on your plan, they "throttle" your connection speed significantly. The intent is to allow you some very basic access such as text emails, but not enough for media sites, videos, etc.
You can wait until your current billing cycle ends, and once the next months cycle starts your data counter will reset to 0 and be back to normal, but if you go above 5gb again, you'll have the same issue.
If you need more speed now, or expect to consistently exceed the 5, your best option is to contact ATT and upgrade your plan. It will cost more, but you can bump up to a 10, 20, 50, etc plan. They may even have some unlimited plans, so you can find what best suits your needs.
If you have any other questions, just let me know. Otherwise, please take a moment to rate my service as this is the only way I am credited for my time. Hopefully I have earned 5 Stars but if not please let me know what I can do to ensure you are satisfied. I also invite you to contact me directly with any future support needs at http://bit.ly/1QwLxGh and ask for Nathan.
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