Something is weird. . .
 

While I was riding today and shifted into a gear, a couple of minutes later the bicycle decided to shift into a higher gear on its own! My bike has a mind of its own! Does anyone know what's wrong?

Yea its fairly common. CHeck out sheldon browns article
http://www.sheldonbrown.com/autoshift.html

Adjust your cable tension....

Check your cable lubrication.

there are plenty of variables here. My advice is just to reinstall your shifting system. By that i mean clean housing and lube them, clean cable, and setup again

Could be a number of things.

If it's shifting in/out of gears by itself, good chance it's just the cable tension. When you shift, a certain amount of cable is taken in/out. If this doesn't correspond to moving the derailleur to a cog (e.g., it moves it between two cogs each shift), then it's likely to go back and forth. This is even more likely if your bike or cable is new, as the cables stretch a bit with use. Read Sheldon Brown's or the Park Tools blurb on adjusting it.

If you've banged the derailleur, it or the derailleur hanger might be bent.

A worn chain can result in the chain slipping in the same gear.