Iva, what's your weather like? Sometimes these symptoms are serious, sometimes easily overcome. Vitamins are good. Nutridrench is my go-to for a quick boost.
How old is she, and is she laying yet?
Would be a good idea to weigh her if you can, and monitor her weight. Try to weigh around the same time each day, or make a note if her crop is full or empty when weighed. And make sure her crop is empty and flat in the morning, before she eats anything.
Maybe try this symptom checker for possibilities.
http://www.poultrydvm.com/symptoms
(may have to hit "retry" at the top to reset the page, I don't know why it does that)