J.R.
Is she really awake when she's screaming? If not, it could be night terrors. My son used to get these on days when he was especially active and/or didn't nap enough. "They" say that you shouldn't wake up babies/children who are having night terrors, but that was the only way we could get him to stop screaming. We would just hold and rock him, and he would usually go back to sleep after a few minutes.
If it's not night terrors or nightmares, it could be teething. Or she may be about to hit a developmental milestone. I remember going through a definite sleep regression at eight months.
Good luck!