I see that your were affected by the double the data plan update and this happened to our customers that are still on a grandfathered plan which means the plan you have is no longer available in the system.
You have a Mobile Share Value plan and it got updated to what we called double the data for an increase of $5. From GB to GB. The reason behind the update is because maintaining the retired plans takes up more operational expenses for them to still exist because they are no longer the main plan AT&T has.