Screen noticeably darker than usual--accidentally changed setting or time to worry?

Hi guys, I've noticed today that the screen on my 2.2 ghz macbook seems noticeably darker than usual--dark enough to make it a bit hard to see (though not yet impossible). I'm not sure whether I could have made any changes in the settings without realising it, but I just thought I'd check to see what you all think about it before I do my usual thing and start panicking

I noticed this first yesterday evening, but especially an hour or so ago when I got home and woke up my macbook which I had Slept yesterday night, as I usually do. The worrying thing is that the screen seems to be getting darker. At the same time, I've noticed some other macbook-misbehavior, including various applications that never before caused me problems with freezing (like Mail or Updates) which did freeze, both yesterday and this morning (I had to force quit them, but then the MB still wouldn't act as it should and so I turned it off and back on again after a few minutes to see whether it would sort itself out. None of these things are especially worrying in themselves, but together, and combined with the darkened screen thing, they do rather bring me to the verge of a full-on 'there's something the matter with my precious macbook' kind of panic

So, any help would be greatly appreciated and suitably rewarded with cookies

Aw hell, I think you're right and this is what I must have done. They're F1 and F2, aren't they, right next to F3 which is spaces/expose right, and to esc which I would have pressed to force quit applications--those are the keys right? So I think I may have done just that. I've been trying to wean myself off of using the mouse with my laptop for the past couple of days, so I'm using a lot of keys that I hadn't used up to now, which may explain why I screwed up like that. Now all I need to do is find the screen resolution I had been using before I also accidentally changed this. God knows how.

Thanks you guys, I kinda figured it out on my own but stil, here's a cookie for your trouble:
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