Severe fatigue weight gain joint pain neck pain nausea brittle hair anemia and good thyroid levels with healthy diet sleep & exercise... whats wrong?

Many possibilities: See a physician - in particular, you may need to be evaluated for rheumatologic diseases like rheumatoid arthritis, lupus, etc. Your anemia should be evaluated, as well -- although many young women have low-end hemoglobin levels because of menstruation.

Don't know: You will need a comprehensive evaluation by your internist. Your description could indicate one or several of many problems.