Battery Light Goes on and off while driving?

A couple months ago I got a new alternator and spark plugs and everything seemed fine. A couple days ago my battery light turns on, then shut off. This happened about 3 times in my trip and decided to take it to a close mechanic. They tested my starter, battery, and alternator and everything passed. Yesterday on my way to school, it turned on once then turned off and then on my way home it did not turn on. Today, on my way to school didn't turn on at all. Now on my way home, it turned on, stayed on for 30-45 seconds and then turned on. This happened 11-12 times in my 20min commute. Does anyone know what could be going on with my car and what I can do about it?

Added (1). I drive a 2004 Ford Focus SE

Added (2). I did take it to a mechanic and he said since the battery light wasn't on when they did the tests they weren't able to find the source, and since my battery light goes on and off when it feels like it, it's hard to get it to them

Added (3). Alternator was not rebuilt, it was a brand new one

Sounds like a loose wire. Go back to your mechanic and have him look again.

The chances are that your new alternator is going bad. The alternator may have been one of those rebuilt jobs. In any case, it sounds like it is intermittently going out on you, and will probably just get worse over time until your battery won't be able to start your car.

Some sensor is malfunctioning.