My t-mobile phone just reset itself..?

My phone's been acting really dumb lately. It replaces letters and symbols in texts, etc. Today, I was texting someone, and one second it was fine, but then when I looked at the phone after I sent the text, I found that my phone completely reset itself. It deleted my outbox messages, reset my wallpaper to default, took off all my app locks, and reset my volume from vibrating to volume 5. It's a samsung SGH-T139. And I bought it from walmart, so I can't take it to T-mobile to get it fixed, cause I didn't buy it from them. How can I fix it?
 
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