The seizures that I witnessed my hasband having started with a very strange screeching noise. The symptoms you describe sound very much like a seizure. My husband did the same thing as far as stiffening but he did have the convulsion part of it with jerking movements. He then fell into a deep sleep. I would say you should definitely get her to a doctor. The can do an EEG and get her on medication if that is the case. Its much better to get on it right away in case it happens at a time when she is alone or out on the street etc. Good luck to you and your daughter. I must say that my husband has been fine since he is on the medication.
I think I found something that might help you. She may have Epilepsy, go see a doctor ASAP!!!!!!!!!!!!
http://www.epilepsy.com/epilepsy/seizure_tonicclonic
I'm not a doctor but its sounds almost like Nonepileptic Seizures. I was reading that there can be screaming. But like I said I don't know and im not a doctor. I would keep an eye on her, if it happens again go see a doctor. Or it might be a good idea to go and see one anyway. Hope this helps.