Brake warning light on?

My brake light was not on until about 2 weeks after my neighbor changed all 4 brake pads and rotors. That's when the light would come on and off while driving. I added brake fluid and after 2 days the light finally went off and stayed off. This was about 1-2 months ago. A few days ago, the light again was going on and off. Since yesterday the light started staying on until I shut the car off. What could be going on? There's absolutely no difference in the way the pedal feels. I can't see where the max line is so I've just been guessing where it should be up to. I haven't added any more fluid this time. I don't know if I should.