This may not apply to girl friends, but people in this situation may not be in the right mind to realize the right reaction if she was a random person would be, "who knows?"
It appears she feels insecure (or other kind of uncertainties or lack of confidence, etc) about something unspecific. A better interpretation would be, she feels insecure about everything in one aspect. It's not about you, and you can't know its source easily, but you merely didn't stop it. Or more blatantly, they may not feel it's better to make some alternative choice that is bad for you (before they begin to feel desperate). You are just the indicator of something that may happen.
This probably applies to most kind of fear when someone has a strong emotional feeling. If the problem source is more unknown, or someone tells them unnecessary to learn, or someone has made sure only the obvious things doesn't hurt while the feeling itself persists, it probably would only makes it stronger. A known threat would probably be much better (before it's known to be desperate).
Think that if you are a nuclear physics, some anti-nuclear people may think you are the best indicator of what can go wrong. While they might be against your ideal and want to remove your jobs, that also means they respect you as the best professional in this field, if this field does anything good. They are personally bad for you if you don't intend to help, but you are not personally bad for them if they didn't go crazy.
Naturally people could have a very low standard for beginning feeling desperate in such situations for all sort of reasons. But before that, it's just a low priority problem, that you are supposed to either simply show them there are no (strong) indications yet, or you have understood it so well to a degree that justifies a one-time-for-all solution, even though there isn't a strong indication indeed. Or for simplicity, just stay calm.
And unfortunately, being able or unable to answer the question directly, or even insisting on its low priority, or even showing your intention to be consistent, is more like showing there "might" be some unknown "potential" problems, which might be trivial problems such as you don't know how to deal with this, or not so trivial problems such as that she cannot understand or you also feel insecure, or some most crazy possible things saw in the dreams.
"Thinking too much" about this, or more exactly, feeling too obligated and behaving like there is an unnatural deadline for resolving it while you think too much about this, is like saying it's necessary to prepare and deal with such extreme cases. And unless you do much more, by common sense she is not supposed to trust you with this premise.
I could say something like "to stay calm usually means to show things works well and to leave the problem itself open." But that could easily lead to someone feeling too obligated to consistently not do anything for it.
The accepted answer should be the answer. But just some more information to make it more logical so that it could be adjusted for situations, and to answer the somewhat clickbaity title. Just another obvious thing: Not knowing how to react isn't anything worse than asking "Why did you do that?" itself.