I'm so late to the game on this.
OK, I have to admit this... I am like your GF. I do the thing where I say "nothing." I am trying to work on it, but it's a hard instinct to let go of.
It's totally a lie. Oftentimes *something* is wrong, but I haven't quite figured it out yet. I'm pretty slow in figuring out the impetus for a lot of my emotions. Half of the time I'm aware that I'm sad/upset/frustrated/angry/bitchy but I don't really know why. And BF gets pissy when he asks me what's wrong and I say "I don't know, I'm figuring it out" and he starts to needle me "Is it this? Is it that?"
The truth is that I know he's trying to help, but for the most part I want to figure it out on my own, digest the problem, process it, and THEN talk about it. It's completely against his nature "as a guy," I know, because he loves me, hates seeing me upset, and wants to "fix it"
I appreciate that, but it's just a different way of thinking.
What I've learned to do, after much discussion, is to say something along the lines of "Something is bothering me, and I haven't quite figured it out yet. I'll be ok, but please forgive my mood. I don't really want to talk about it right now, but if we talk about something else I'll try to set aside my mood."
Sometimes it works, sometimes it doesn't, but it took him nigh on 6 years to train me to say that (I'll admit, I've been trained). It sounds like with your GF that she wants you to read her mind. I admit I do that too. I think "He should KNOW me by now, and what tends to bother me, and figure it out!" but in all honesty, I can be pretty obtuse.
As a for instance: The other week I had mentioned how I'd love to have plants in the house. He said something to the effect of "Me too, but we tend to kill them." I said "That's true, but I'd like that to be different this time." He said "Okay, cool."
Fast forward to a few days ago, when we're walking by plants in the grocery store while we're out on errands. I say something like "Oh, look, these flowers are nice." Of course, I'm thinking about the convo we had ten or so days ago. I'm HOPING that he will say "Oh, yes darling, those are beautiful, let's buy a dozen!!" (or something to that effect). Instead, he, being distracted (he was looking over our errands list) kind of glanced up and went "Huh. Yeah." Totally un-enthused.
There was NO WAY he could know that I was hopefully scripting a response in my head, but my immediate emotional response was to get pissy that he did not conform to the BF-in-my-head-hopes that he was as excited as flowers as I was. So I got a little angry. I knew that the anger was misplaced and was about something I hadn't figured out...so in effect, I knew I was being crazy so I tried to push it aside, but that takes me a few minutes to do, usually.
He noticed I seemed off in about 30 seconds and asked if I was ok. Me, knowing that I just needed a few minutes to get over my own crazy, said "I'll be fine." Proceed to tense, awkward conversation where I eventually said something like "Well, we had talked about buying plants but obviously you didn't mean it!" (<--- that's some crazy coming out right there!!)
Of course, what I meant
to say was "I had been hoping that you would remember our previous conversation and point out some flowers, thus letting me know that the little things are important to you, and that you remembered how much I wanted to try again." However, I hadn't figured all that out yet, all I had was this vague emotion of feeling let down and disappointed. And, to be fair, there was no way that he could know what I was hoping for, unless he was a mind-reader. Instead, I got this as a response...
Him: "Uh..okay, well....buy the plants if you want to buy the plants, babe. They're nice."
Which just reinforced the feeling of him "not caring" about the situation to me (that is, that he really didn't care about what I wanted), when in reality he was just trying to "fix it" and, believed that the whole problem was was about buying flowers. So, he was giving me permission to do what it was I wanted to be happy. That's a guy thing --> do an action, fix a problem! All better!
But what I really wanted was validation (which he had no way of knowing), and I felt like he'd pushed the whole thing away from himself. Of course, this is a stupid argument over buying flowers, but I blew up with something like...
Me: UGH YOU DON'T REALLY WANT THEM DO YOU?! WHY CAN YOU NOT BE ENTHUSIASTIC!?!?!?!
These sorts of things happen all the time. I agree with Cassie and mhibidon -- she might not even realize she's doing it, OR she may be expecting you to read her mind, OR she might be holding a grudge from some earlier behavior you didn't even realize you did. It could be a thousand things. All I can say is to be patient, realize that she might not even realize what's wrong, and let her know that you can't read minds and that you DO appreciate the little moments in the relationship from time to time, because women blow those things up sometimes