Hi Caitlin
to go from 2 1.5 hour naps to 1 nap is a heck of a jump for a lo- from roughly 3hrs A time to 5/6 hours in one go. The short nap is probably due to being overtired.
We're currently transitioning from 2 naps to 1 also. There's a bunch of us on the 2-1 Nap thread at the top of this board- come & join is! Most of us there have started by cutting down the morning nap, ranging from 20-45 minutes, to ensure a good afternoon nap. Those who have started trying 1 nap a day are finding that their lo's can handle 1 nap for maybe 2-3 days, then they need a 2-nap day to catch up on some sleep.
It seems your dd is definitely ready to start the transition, but maybe it needs to be more gradual for her.
hope this helps
kathryn