When you change out your pads, it is necessary to also change out the sensors (which is basically a metal contact with a cable attached; a $10 part. You need 2). They are typically fused to the old pad over time and if you tried to remove and reuse them, they would most likely break.
The sensor slots in to a groved cut-out on the brake pad. If the sensor is broken, it might fall out of that cut-out and touch the brake disc - triggering a false warning.
Your warning light could be either of these 3 things...
- Your brake sensor is touching your disc due to improper installation or reuse of a broken sensor.
- Your brake fluid is low.
- Your parking brake switch has shorted.
Check all the above, if everything checks out ok, it is possible that the warning light stays latched for a while, and would go out on its own after a couple days.
If it still doesn't go out after a few days, go to your local AutoZone or Advance Auto Parts store and ask them to plug in an OBD2 reader to see if they can read/reset any fault code/s. They'll do this for free.
If that doesn't work, the last resort would be a drive to the dealer.