Why does my iPhone 4s shut down immediately when I play a big game?

When I play games like Gangstar Vegas, or Dead Trigger (but also with smaller games like offroad legends and beach buggy blitz), my iphone 4s suddenly shuts down 30 secs after I launched the game EVERY TIME. Tried restarting it, leaving it shut down for like a day, but nothing. This is the first time this is happening, I used to play these games for like an hour just fine, without lagging or shutting down. Now the only games I can play, are small games like Temple Run, Subway Surfers and Worms 2. I'm not jailbroken and on the iOS 6.1.3. I'm f*** desperate.

Because its an apple product. Get an android.

Here some tips for the iPhone. Is you turn on the phone by hold the power button and the center button. It will reset setting with out loosing data. You can close running apps by double hit the center button. Then a line of apps will appear at bottom. Press and hold an app to make a sysbol appear. Then press it the red sysbol to closes apps. It won't delete but your phone run faster.

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