First... Welcome to the forum!
I've heard of this with other brand pistols before, but not the Shield. In those instances, it was one of two things (with regular pressure ammo)...
1) there were signs of a slight burr either on the frame rails, or on the slide itself. Once the burr was smoothed, no more problems... Or
2) The Ejector (not to be confused with the Extractor) got bent behind the breech face. THIS though,would not have been freed up with only a couple taps to the rear of the slide.

Since you were able to run another 124 rounds down range after this incident, there's the possibility that there may have been a slight burr that hung yours up that got knocked off when you smacked the rear of the slide. Field strip it and check (feel) the slide and rails for any irregularities. If you can find signs of a burr (or that there WAS a burr that got knocked off), you at least have something to mention in a follow-up call to CS.

If it's your ONLY carry pistol, I wouldn't carry it until it's been tested some more (or sent back for a check), but I wouldn't be so quick to send it in. I'd run it through a mini 'torture test' of sorts.
I'd run at least another few Hundred rounds of FMJ through it to see if it happens again... Quick doubles, triples and 5-7 bursts of rapid fire (if your range allows it)... don't be easy on it. If it's Jam Free after another 200-300 round 'torture test', it could have simply been a fluke that might never happen again. THEN I'd start experimenting with various brands of JHP carry ammo to see which your pistol (and you) like.