Macbook screen Goes Black after a few minutes

I recently had my Macbook Pro 2010 13" model repaired with a new hard drive since the last one failed when I got it repaired from a dodgy Apple re-seller a year ago.

When we ran System Diagnostics at the Apple Store they said the only thing they found a problem with was the old Hard Drive. Everything else on the screen indicated that it was working perfectly I now have a SSD which works great, instead of a Hard Disk. But now when I leave my computer running about 3 minutes later the screen turns black. It's not a big deal but I just keep thinking that the computer turns itself off when it didn't. It goes black every time I leave the machine. However, in Preferences, Put Hard Disk to sleep whenever possible is unchecked, and I have turned the screen saver off, but the screen still goes off. I have tried looking through many different online sites about this issue but none have come up with any acceptable details or solutions. Please help.