Why is my iPhone 4 really slow?

I don't know about phones but I hope I can get some help on what's going on.
I have an unlocked iPhone 4 3G from T-Mobile I have the unlimited plan (which is 60$ every month). I've been having it for 2 months & it's been slow since I've had it but ever since yesterday it's been extremely slow. It keeps getting the "E" symbol (what does that mean btw?) & it says no service (I just made a payment 2 weeks ago) it only works when I go to the 2nd floor of my house. I don't know what's going on it's really frustrating. Oh & I don't know if this info is important but I also got the 4GB data & when I go to acellular Data Usage it says 7.9GB. I don't know if it's that. Can anyone help? :/

Try clearing internet histiory

Hello the latest version ios 7 on iphone 4 is really slow yes.install fresh firmware, via itunes, if you have problem with no service contact tmobile center and replace phone
The "E" symbol is your data network EDGE (Enhanced Data GSM Environment) for surf on the web chat etc and speed is slow on EDGE until you get 3G symbol, and yes you have used 7.9gb data network when you start to use iphone reset statistic each month to control your usage cuz you have 4gb, you will pay more if you use more GB chers

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