I was thinking as I read your post -- about not remembering the mood swings when you were this age...I don't know about you, but there are lots of times when I get cranky from PMS, or when I was pregnant, and had no idea til after the fact (and usually not til someone pointed it out to me). You might just not be remembering it! :-)
It has to be the age. Believe it or not, my son has been emotional lately, so it's not just the girls. It's a change in life phase, or the openings of...isn't it great that she feels so comfortable as to share those things with you?
One thing we've started to do with my son is to have him write down things he's worried about, or are stressing him out. WIth him, he's usually more focused on if he's done something wrong, but you could do the same with your daughter for what's making her weepy. And then we have him put a check by what is really worth worrying about, and what needs to be let go. This (we hope) will help him identify what is a valid worry, and what is just self-inflicted and unnecessary anguish. And then once we go over the list together and talk about the important stuff, we let him know that it's time to let go of everything else.
He's really responding to it, and maybe something similar could help your daughter recognize what is healthy to fret over, and what is not. Good luck!