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When I used to make phone calls, the screen would go black when it approached my ear, and now it doesn't anymore, making me hit buttons and things when i don't want to. For example, I am always hitting the mute button. It has nothing to do with my case because I don't have one. Any idea's how I can fix this problem?

There may be a problem with the proximity sensor. To troubleshoot this you'll have to:
Create a backupImport your camera roll photos and/or videos to your computerRestore your iPhone as new
After you've restored your phone, when prompted selest "Set Up as New iPhone" and then make a few calls. If the display still won't go blank the proximity sensor is malfunctioning and you'll need to get it repaired. If the display does go blank then it's a software glitch, so if you restore the phone to your backup you'll continue to have the same problem.
How to backup: http://support.apple.com/kb/HT1766
Importing photos and videos to your computer: http://support.apple.com/kb/HT4083
Rs=estoring iOS software:http://support.apple.com/kb/HT1414

Before restoring the device, remove any cases or screen protectors on the device to rule that out as the culprit.