You haven't changed your plan but you changed to a different style phone that uses WAY more data. If you were still on the old "dumb" phone, you wouldn't be using near as much data, but even back then in the terms of service, there was a rair use policy that AT&T could have envoked and limited your data back then. But because no one was using that much data, they didn't care. When smartphones came on the scene, data use jumped exponentially, causing the provider to scramble to keep up with the demand. By throttling huge users, using the fair use policy, it helped with the demand. Even with the upgrades to towers and adding LTE, and with the addition of huge numbers of smartphones, you either need to pay for the data you use, so that the provider can afford to upgrade towers, or soon no one will be able to use data because of the overload.
Those old data plans did not expect people to use GBs of data per month.