babies go through TONS of developmental changes... and their REM sleep is adjusting too or gets interrupted, and sometimes for no reason. It's all physiological or developmental.
They don't have "nightmares" because something "bad" happened to them... developmentally speaking, they will later have "night terrors" (you can research it online) and later "night mares." Night terrors and night mares are 2 different things, which occurs at certain developmental ages.
Babies make all kinds of noises as they sleep... or when transitioning in their sleep to various REM patterns/deep sleep/light sleep levels. MANY times, they will "appear" to awake and make "cry" like noises... but this is just transitory... if allowed to, they will self-soothe and go back to sleep. If she is going back to sleep on her own... then let her. This will encourage good habits on her part, to learn to self-soothe.
It's perfectly normal....both my kids did this as babies, and even now (daughter is 5 years old, my son is 22 months old), my kids make all kinds of sounds during sleep. My girl even talks in her sleep sometimes. Even adults make noises in their sleep too!
Your girl is on the cusp of a growth spurt (6 months is a growth spurt time), and she is probably going through lots of changes now. Their sleep patterns change too, and they will often get hungrier too at growth spurts.
My girl for example, was also very cerebrally active, even in her sleep... and she was a light sleeper. But even with her transitory sleep "noises" she was fine... it was just her and the way she was.
But do read up on "night terrors"... as your girl gets older...closer to 1 years old, this is when it can crop up. It's NOT harmful, just the name of it sounds like it. And it is purely a developmental stage, which is temporary.
Take care,
~Susan