Middle of the night screaming

S turned one a week or so ago. We're still co sleeping and i'm night nursing for lack of ability to wean her. She usually stirs a few times in the night, and if SO and I are still awake he goes in to soothe her. If we've gone to bed, she lunges at my boobs, sucks for a while and falls asleep all cuddled up next to me. That is, until she moves around a little in her sleep and ends up spread eagle in the middle of the bed for some reason. But I digress. this morning at 3 or 4 she woke up SCREAMING her little lungs out. I tried to nurse her and cuddle her and hold her and rock her but she just kept crying. Checked her diaper, nope. Too cold? Too hot? Nope, nope. Limbs asleep? -pokepokepokepoke- No change in crying when I poke. We sung her favorite song, "Oobla Di Obla Da" and she fell asleep. A few minutes later, crying. It was like she was 3 months old again. The rest of the night was a blur, I think she fell asleep cuddling. It went on and off until the break of dawn when she got up.
After she was sitting up and reading in bed and looking out the window at the deer in the field, I passed out cold. I was woken by SO changing the most dreaded morning diaper. He put her back down and went out for a cig. I was woken a little bit later by S staring at me in inch from my face. The muffled sound of a baby gate in the hall startles her in a happy way. I take her out to the living room and find SO. He was about to go get the baby to go for a walk so I could sleep. Aw. Oh well.
What do you make of this sudden middle of the night screaming? We thought maybe a nightmare? Or something?