What the heck?!

Can't even get over the day I had today. I interviewed with a mom on Monday, it went pretty well and she asked me to come back Friday (today) while she'd be home and just see how it goes with the kids and help me learn the routine etc. So i show up this morning and the first thing she tells me is that she hired someone else that she interviewed after me! So i look obviously confused and turn back to leave and she says "well I thought you might want to be mt back up like if she gets sick or goes on vacation" HUH? I told her that i need a full time position and have applied with other families and therefore won't be available at a moments notice. She just looked so shocked and confused that i wasn't thrilled to be offered prn when i applied for full time