Baby Sleep Cries - Help!

Sara
My baby girl is 6 months old and is the happiest, easiest baby ever. I'm really not trying to brag, and I feel so guilty talking to my other mommy friends because she is seriously so easy. She sleeps through the night since she was about a month old, and she hardly ever cries, and when she does, its with her mouth closed, and more like complaining. 🙄 lol (kinda hard to take her cries seriously when she's that quiet lol)
But lately, she has been crying out in her sleep! Daddy talks in his sleep like a crazy person, so sometimes I think it's like her way of talking in her sleep.. But it's the saddest sounding cries & the most heartbreaking noises! And since she really doesn't cry much, even in the mornings, when she does, something is wrong. So I keep waking up several times a night to check on her but she is sound asleep. She is also tossing & turning in her sleep, fidgeting and fussing around. What could be wrong? Could she be having nightmares? She is in a bassinet in her room, for now, maybe she has outgrown it? I moved her into my room tonight and she won't stop squirming & wont fall back asleep. 
I'm sorry that was so long! I'm so worried and not sure what to do or if I can do anything. Does anyone have any advice?
Thank you!