Problem with ATT iPhone4

So my g/f and i are on the same plan together (cept i have unlimited data).

I have a 3GS and she has the 4.

I have had zero problems with my phone, but for the past week, her iPhone 4 has been on the "E" network instead of the "3G".

Is this an ATT thing or is there possibly something wrong with her phone?

Im pretty technically smart, but i cant seem to figure it out.

Thought id ask here first before going to the ATT store.

Thanks for any answers you guys (and gals) can provide!

-Sin

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This probably isn't the case, but is there anyway that 3G could have gotten turned off? To check: Settings-General-Network. Good luck!