Did you have a double data promo plan that required a 4G phone on the line? If you did, and you activated a 3G phone for any length of time (over 14 days) you would lose the promo because the phone doesn't qualify. If you put a 4G phone back on within 14 days you can request to change back to the promo, but if it was longer, or you didn't put a 4G phone back on the line, you lost it. Some promotional plans have stipulations like that, and while you should have been informed of the change, you also should have been aware of the requirements to keep the plan.

Whenever I make changes to my account, I check my plan and features and things online after a few days, because a lot of the time you won't get a bill reflecting the changes before that 14 day window and I want to verify that the changes I requested (and only those changes) have been made. Best of luck to you!