I suppose it could be overheating, as Randy said, but the closed eyes and wings spread sounds more like she's stressed. Little Ju was vomiting a couple of months ago, too (he's fine now. He must've eaten a bad sprout or something). He was retching like your girl, and he had has eyes closed and wings spread, too. Budgies sometimes close their eyes, spread their wings, and tremble in response to stress, and few things are more stressful than vomiting.
Seeing as there aren't any avian vets around, you'll have to treat her holistically. I'd definitely separate her and put her in a room far away from the others. You should give her Bragg apple cider vinegar, or any other ACV with "Mother" in it, mixed into a fresh dish of water. You should also mix ACV with "the Mother" in it into your other budgies' water.
I'd also make up a cup of ginger tea using a fresh, chopped up ginger root. Pour hot water over it, then serve once it comes to room temperature. She might just be vomiting for no reason, or it could be sour crop, yeast, or bacterial. If it's related to yeast or sour crop, the ACV should fight the little nasties, and the ginger is antiparasitic, antibacterial, antifungal, and should help soothe her tummy and boost her immune system. If I recall correctly, cayenne is antibacterial or something like that, and also purifies the blood and improves circulation. I sometimes mix cayenne in with my kids' sprouts.
You should also give your other budgies ginger tea in case it's infectious. So, in both cages you should have water with ACV in it, and a dish of ginger tea that should be thrown out, washed, and replaced with fresh tea every three hours or so. You'll also want to disinfect the cage they're in. If you don't have any bird-safe disinfectants, you'll probably want to use diluted bleach and put it out in the sun for a couple of hours, then thoroughly rinse it. Obviously you don't want the birds anywhere near the cage while you're cleaning it.
Care to share their diet and whatnot? Oh, and I buy dried lavender from the health food store and keep a dish of it in my guys' cages at all times. It helps with anxiety and stress, and they eat it to help them stay relaxed.
Samantha, Charlie, Ju, Rumi, Lara, Pollo, Ziggy, and Alice
Rest in Peace
Sam, Nova, Joey, Maria, and Scarlet (March 4, 2014)