Our DD (now 11) acted extremely similarly when her school situation was inappropriate. While allergies, illness, etc. can also be a factor, I'd take a closer look at the school. Here are a few things to think about:
Intellectually gifted doesn't necessarily mean having additional physical maturity. The long days might just be too much, particularly if the academic level is still not appropriate. So she went from "relax and play all day" to "have to sit still and work and work, but not on anything interesting."
K - 2nd is a time of major physiological changes. When our DD was in these classes, we needed to constantly advocate for things like snack time (so she didn't crash and flip out in class -- you'd think teachers would understand this sort of thing).
Some of the things you describe sound like perfectionism and performance anxiety. Maybe being in the higher classes has given her the idea that she has to be perfect in all areas, or that there's something wrong with her because she's still not happy, or that by complaining she will disappoint you after you got her into this special situation. If she was anticipating that this would finally give her something interesting to learn, it might just be crushing for her to realize that that's not the case.
Finally, she could be feeling abnormal. It's hard doing things that are different from the other kids and it's hard being younger.
KathrynH had a very wise point about a perfect storm of factors contributing to this as well.
