My daughter is also 18 months old and does the same thing. She either a)takes a two to three hour nap and wakes up happy and refreshed, or she b)wakes up after one hour and has a complete, inconsolable meltdown. She screams bloody murder and doesn't want me to hold her, but if I try to put her down, she cries even harder. When she is napping, I keep my eye on the clock, and the one hour mark, I listen for her very carefully. If I hear anything, I try to respond to her right away. The *only* thing that calms her down is to put her in her sling and go outside. If I do it quickly enough, she will sometimes go back to sleep. If for some reason, I don't get there in time, she won't go back to sleep, but the change of scenery seems to soothe her. I have had a feeling it was bad dreams for a while. I think the one hour mark is when she is having her REM sleep, and if anything wakes her up during that time she still has the dream in her head and it is very disturbing.
Today she did this, and when I went to her, she looked very frightened, and said, "FALL DOWN!!" This was the first time she has ever used words in any of these episodes. It kinda confirmed my bad dream theory.
I was able, BTW, to get her to go back to sleep and she has been asleep for nearly three hours.