Is it normal my girlfriend just started crying in my arms over 'nothing'?...

Just to let you know, she doesn't have to be on her period to have PMS symptoms. And she doesn't have to be PMSing to be emotional.

I did this with my boyfriend once and it was because of something he did in the past that I hadn't gotten over.

She probably represses her feelings that bother her so much that she might not know exactly what is bothering her. I doubt she is trying to hide something from you specifically but it is possible. It could be something that happened to her in the distant past or something she's currently going through that she doesn't want you to judge her for (like molestation or abuse). Just comfort her and don't be suspicious. You have to gain her trust for her to open up to you; depending on what it is it could take years.

Did a certain part of the movie remind her of something in your relationship?

When she says it's "nothing", it's always *something*. But don't push her to open up to you if she isn't comfortable or ready.

If and when she does spill, don't -- under any circumstances -- underestimate or trivialize the topic. If it *is* something you did that she's hurt about, don't expect her to "just get over it". Healing can happen quickly, or it can take much longer. The best thing you can do is give her as much time as she needs to feel good again; otherwise, said issue might become a permanent chink in your relationship armor.

Girls are emotional creatures.
While it's uncommon, they can sometimes have random bursts of emotion.
Whether it's something subconscious that she's not aware of yet, or something she's not comfortable sharing, only time will tell.
Just comfort her every time and she will bond with you better.

Also, there could have been an emotional scene in the movie, or one that she could personally associate with.
As a guy, I even experience this sometimes. (but not to the extent that I need to cry over it though.)

Women experience hormonal imbalances at times, which makes them prone to mood swings. It's likely that's what it was.

That or she is going to dump you but doesnt know how to and its tearing her up inside

Well it's probably not nothing, she's probably just not telling you. And no I don't know what it is, it could be absolutely anything!

Unless there's a hormonal imbalance, people don't cry for no reason. I'm sure there is something that she's keeping to herself. It may not even have anything to do with you. One time I was crying in-front of my boyfriend about something bad that was happening in my life. Now, I could have cried alone but I cried in-front of him because I wanted comfort. There also could have been something about the movie that triggered it.

No thats normal. Girls don't have to be on their periods to be hormonal... I have breakdowns all the time. It happens every now and then, but I try my hardest not to let it happen in front of people. If I feel like I'm going to start crying, i go have a shower or something. Honestly, girls can regularly just get random floods of sadness, like, we just start thinking about life and how all these different things suck and there's no /specific/ thing making us sad, its just everything, and we have no idea. She's probably not hiding anything:)

You never know what has or does happen behind closed doors. If she doesn't tell you, don't freak out. She will if when she's ready.

All you can do for her, at this stage, is hold her and tell her everything is going to be okay and that you are there for her.

its normal. girls do that all the time. i woudnt worry about it unless it keeps up she may be hiding something from you. just try to be there for her but yeah. be careful about emotional flakes though.

Guilt

What movie were ya'll watching?

Baggage. Run.