Pain or nightmares?

One of my gerbils has been doing this thing every night for the past week sometimes a couple. She screeches at the top of her little gerbilly lungs and this has happened when nothing is around the cage so it's obviously not a disturbance even though I run to make sure anyways! She is never caught on anything. She is usually huddled up in her little Carefresh fort. The sounds reminds me of when my ferret SCREAMED three times in the middle of her sleep. She was asleep and we assumed it was nightmares. Could the same thing be happening with Weeping?