Air in the brake lines.

Air should not get in there doing a normal brake job (replacing pads) unless someone disassembled your calipers or disconnected your brake lines. But whoever did that should have bled the brakes when reassembled.

It has been awhile since I did that, but there should be a bleeder behind the caliper, and it works best if you have a clear tube attached so you can tell when air or old brake fluid is flushed.

Have someone step on the brake (and not release it) as you open the bleeder. As the pedal goes down, close the bleeder before telling them to release the brake pedal. Repeat as needed.

If you're going to do it yourself, pick up a Haynes or Chilton manual. You'll need to remove the fuse for the ABS and bleed the brakes in a particular order. It's all spelled out in the manual.