I have been vomiting and nausea for about a week now. my back wont stop hurting and today my head is pounding. what could it be?

Gastroenteritis: This is probably a viral gastroenteritis. If the timing is right then do a home pregnancy test. Whatever fluid you lose from vomiting replace with equivalent amounts of water (sips of fluid, a little at a time). Most of these are self-limited and will resolve on their own. Replace whatever fluids you lose so as not to become dehydrated. If these symptoms continue see your doctor