ABS light remains on

I recently changed my timing belt, fan belt ,water pump and front brake pads. They were all working fine. But after a week a new problem cropped up. The ABS light remains on even after the start up, i made sure that my hand brake was fully disengaged, but the problem persists. After driving for a while the light goes off and then a while later comes on again... The brakes are working fine and i even tried E- breaking and all seems fine.... what could the problem be...? Pls help.....