Why am i so tired (im 16)?

You should stay well hydrated, and you should probably ask your doctor about this. They may want to do a blood test. Several very simple and easily treatable things could cause the feelings you described. For example, having low vitamin D levels can cause irritability, depression, weakness, pain in your limbs, and general fatigue. But it can be treated and fixed in a matter of weeks. Speak to your doctor. Best wishes.