Was it too hot in the trailer? If trailer has been closed up the build up of hot air is enormous, and is enough to make a horse balk at loading.
And might want to check your trailer over really well, as horse could be reacting to something wrong with frame or flooring.
Did you make any sudden stops the last time you hauled or or time before?
Could be something is flapping in her face. Dirt from road, window rattling, lead rope? Get someone to haul you around in back of trailer for a little bit, and see if anything pops out at you.