sleepwalking dog

Hello,
   
I have 2 teacup Yorkshire Terriers, they are 1 year old and from the same canine. They are with us since they were 6 weeks old. My smaller dog is having nightmares for about 7-8 months now, usually just once a month, when she starts barking and attacking the other dog, who's screaming, so I have to separate them but it is very hard. She seems like she is sleepwalking with her eyes open, but she is not awake, because when I wake her, she seems surprised and doesn't know what happened. The other dog seems to know that she is just had a bad dream, but I'm really worried, especially now that is happening every night for the last 7 days. Usually she is very calm, not agressive at all and never barks.
   
Did you ever heard anything like this???
   
Please let me know what should I do.
   
Please help!
   
Thank you,
   
Suzanna