2009 Hyundai Sonata jerking before coming to a complete stop - dealership that did the diagnostics were worthless.

I have a 2009 Hyundai Sonata with an automatic
transmission. A couple of weeks ago I noticed just
before coming to a complete stop the car
jerks/catches a bit, around when the car would be
downshifting from 2nd to 1st gear. The car shifts
smoothly when accelerating, and if I let the car roll
to a stop without hitting the brakes it doesn't jerk
either.
I figured it was either a front brake problem or a
transmission problem. I just dropped the car off to
be serviced at a Hyundai dealership, and they just
called for me to pick it up can't tell me what's
wrong with it. They said both my front and rear
brakes were fine. My transmission fluid was pretty
dirty, so I had them flush it. They said the car isn't
throwing out any transmission error codes during
diagnostics, and they said there is no way for them
to diagnose what is wrong with it otherwise. So
basically they're telling me, "Not sure what's wrong.
You can come pick up your car that is still broken
now. Good luck!" (?)
Any thoughts as to what could be causing this? I
am a woman with limited knowledge about cars,
and shops often try to bend me over for all they
can. I want to take it to another shop for a second
opinion, but I don't want to sound like an idiot when
I do.