I have recently lost a lot of weight aswel as feeling tired dizzy no energy and weak. what could this be ?

Differential: The "differential diagnosis" is broad. The first thing is to go see your doctor. This won't be something you can diagnose yourself. Here are a few possibilities: - autoimmune thyroiditis - where your immune system attacks your thyroid. - gut malabsorption problem, including gluten sensitivity or "sprue" (or celiac disease) -cancer, god forbid. (but you're young, so this is less likely).