Baby's bad dreams?

DD is just over 4 months old and for about a week now after i put her to sleep of a night she sometimes crys in her sleep, It's such a sad cry it nearly makes my cry! I go and check on her and she's always asleep.....She is a great sleeper though, she sleeps from about 9pm till 7am and has since she was 2 weeks old. So i guess my question is, is she having bad dreams? or is it something else?

It could be bad dreams. She could also perhaps be having little growing pains or teeth just starting, not enough to wake her up fully yet?
At 4mths it's like they wake up from being a newborn if that makes sense. Alot more interaction and noticing things around them. Getting into a routine of sorts. So it could be dreaming of all this.