Mine wakes up screaming, and I wonder if she could be having a bad dream. I mean, is that even possible?
Nightmare on Crib Street
It's possible your baby could have nightmares. We do know that baby sleep contains a lot of REM (the stage of sleep during which most dreaming occurs). What a baby dream might look and feel like—a barking dog, a mobile, the startle of a stranger's face from earlier that day—we don't know, but there's no reason to think babies can't dream.
Bad dreams are more common in the preschool- and school-age years, though. So let's think about other reasons your baby could be waking up crying:
1. She's over-tired
Sometimes when babies are a bit sleep deprived, it produces a stress response and they wake up crying. If that's the case, I'd suggest putting her to bed a little earlier—or making sure she's napping enough. (You can see a chart for all baby and child sleep needs on The Happy Sleeper website.)
2. Fallen Pacifier
She falls asleep with something (like a pacifier, or your presence) right there, but when she enters a light sleep and stirs in the night, she realizes it's not there. It's unsettling to wake up without your paci or your parent, if that's the way you fell asleep. In this case, you want to work on her falling asleep in conditions that stay the same throughout the night.
She's not fully awake when she's crying. Sometimes babies and little kids make noises and cry in their sleep—for example, when they have night terrors.
If none of the above are true, and she's not in any physical discomfort, it might just be that she really needs something, like a feeding, and she's just got a clear and loud way of telling you about it.
If you do think she's having a nightmare, comfort her until you feel her calm down. Then try to place her back in the crib awake, so she can do the falling to sleep part on her own.