My anti-lock brakes light is on.?

I dont know why all I did was install a set of wheels on it. 94 Mustang GT. I took off the orig 17x8 wheels and installed a set of weld racing wheels on it . The front are 16x3.5 the rears are 16x8. I didnt modify the calipers or touch anything. The light came on only after I drove over 50 mph. I drove it around the subdivision at 25-35 mph and it didnt come on for 2 days but as soon as I go out on a street were I can go over 50 the light will come on. I'm confused. I switched the fronts back to the orig. wheels and light dont come on.