Battery Light 97 Corolla

I have a 1997 toyota corolla, for the past 2 weeks my battery light comes on for a couple seconds and goes off all throughout the time I am driving it. Some days it comes on more than others and some it stays on longer than others. When the battery light comes on all of the other lights in my car (ie. Lights in the dash and headlights) get brighter as if power is surging in them and the horsepower my engine puts out gets reduced. When the battery light goes off the rest goes back to normal. We have checked the fluids and all are good, we have checked the battery and it is at 13.3 and the alternator is charging at 14.5. Does anyone have any idea of what could be causing this?