Sudden nightmares/night terrors

So I have a stepdaughter, I actually have several but we are gonna talk about this one specifically. She’ll be Kelly for this story. So I met Kelly shortly after she turned 3, she is now 8. I obviously don’t know all the ins and outs of her life with her mom but I’ll give as much information as I can. Her mom and dad split when she was 6 months old, so she only knows mom and dads don’t live together, her moms always had a boyfriend and then I came around when she was still pretty young. Therefor I don’t feel she has to much trauma from the divorce. I know at her moms house she still sleeps in bed with her mom, and when she stays over at our house her dad will sleep in her bed with her. If for some reason she falls asleep by herself she’ll wake up in the middle of the night and come in our room and tap someone awake till her dad goes to her room with her. We have lived in a couple apartments, iv seen her have a nightmare or two nothing out of the ordinary. Feel it’s normal for kids, especially with ones who have unlimited unmonitored YouTube and older siblings go show you scary movies. We have been in our current apartment 2 years now. Iv never had an issues here, my son who woulda been 2 and a half when we moved in. He had like a weird week of nightmares sometime last year. I really monitored what he was watching on Netflix/Disney and nixed some shows and he’s been fine ever since. I don’t think Kelly has ever had a nightmare at this house up until this story. So last week she was spending some extra days here, nothing odd. She’s taken over her sisters room here, been that set up for a few months now. Furnitures all the same but she got to pick out some new bedding add a few toys. According to my boyfriend cuz I can sleep through anything, like middle of the night he woke up to Kelly just bawling. Not crying like kid scared crying and he went in her room and she was awake just staring at something on the wall just hard crying. Obviously he turned on all the lights and the only thing hanging in her room is a fan so he took it down and laid down with her the rest of the night and she was fine. Well now again I was asleep but my boyfriend doesn’t work tomorrow so he was gonna stay up and let the kids stay up. He said Kelly fell asleep on the couch, not odd she’s like 8pm bedtime kid my sons like a 1am bedtime kid. Normal for us, he said then Kelly got up and was smily laughing arms out like she wanted a hug walking towards him. He was in the kitchen off the living room. Again not weird she’ll knock out on the couch but get up 10 minutes later. My boyfriend then walked to meet her halfway and turn her in the direction of her room kind of like “ya your tired just go to bed.” He said when he got to her though her eyes didn’t look right, like she wasn’t looking at him. Obviously super chilling, she’s never slept walk, but what ever he gives her a hug and turns her around and tells her to go to her room. He said then she took a couple steps back in the direction of the couch, her bedroom door is on the other side of the living room. Took maybe 2 or 3 steps and just fell like totally hit the floor not a trip or anything and then just started screaming. This woke me up a bit but I honestly thought it was something on the tv, like everyone else was up all the lights where on my mind didn’t go to kid having a nightmare. Obviously my boyfriend ran over and got her off the floor and he said she just kept repeating “mother”

She’s never called her mom mother in the 5 years iv known her. My boyfriend said he called her mom but she couldn’t even calm down enough to talk over the phone. Again her eyes were just deadlocked on something on the wall and she was just trembling crying unable to speak. My boyfriend ended up taking her over to his moms and he said soon as they were out of the house you could feel the tension leave her body.

Her mom said she hasn’t had anything like this at her house. We’ve been in the same house for 2 years nothing has changed. We don’t even have things hanging on the wall. Nothing traumatic has happened recently to her that her dad or I know of.

Just extremely strange.

Anyone have a similar story or any advice?