And some people think those of us who don't think private medical records will stay private under Obamacare are wearing tinfoil hats.
I'd guess that your insurance company, or the mental health facility, reported your "visit" to some agency or directly to the NICS system. But they should not have, and (unless CA law differs from Federal law) you should not have been flagged. The Federal law, as reflected in the question on the Form 4473, is if you have been INVOLUNTARILY committed by court order for treatment. You entered voluntarily, not by court order, you were not committed, and you were there for observation, not treatment. Three strikes.
I would go over that with Yvonne if/when she calls back. And also ask her how to get your records cleared if what she's looking at indicates anything different.