You screwed up by offering too many details. HR people aren't looking for a reason to hire you, they're looking for a reason NOT to. Next time, if asked just say that you left your last job to try and get into PA school, but that it didn't work out. Leave it at that.
Yeah, this. In your case, she heard: I'm looking to further my education which might mean I'm a short term hire. Or, if I continue my education while working, I'll expect pay raises.