Take a real close look at all four of your valve stems. My shop once tore off the sensor package that is inside the wheel when they were popping the bead and unmounting the tire. They owned up to it and replaced the sensor, about $120. If the guy that did your tires ruined a TPMS unit in the wheel, maybe they pulled a fast one on you and put in a regular valve stem to hide the expensive mistake. Take a look to see if one of the valve stems is different from the others. That may be a clue.