Well, I'd just say it's likely your meter that's the main culprit, neither a glass of water or a greek salad really is that strong a hypoglycemic agent.

As for the metformin, I haven't heard of it having that kind of effect, but I don't take it, so I'll leave it to others to make a comment if it applies.
Probably worth thinking about what you really were. 313 is pretty high... ketosis most likely kicking in, possibly getting lethargic, peeing a lot, feeling off, that kind of thing. Did you feel that way? Can you normally feel side effects when you are that high? If you've been under good control lately, I'd have thought you might feel some effects with a high like that, but you're the one in the know when it comes to your body.
Anyway, when I suspect that my meter is not being accurate (well, as accurate as we expect) I pull out a backup to double check. Do you have another meter? It's a great idea, especially if you lose one. If you don't want to get other strips, get the same brand of meter.