have a question about at&t samsung flight 2?

just got the samsung flight 2. have the unlimited texting/multimedia messaging on it for $20 a month. now to the question, sometimes in the upper left corner there is a blue "3G" and other times its a blue "E" what do these mean? and am i getting charged for something by those popping up? (as you can tell im not too phone savvy) please help? :)
 
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