I have a non stop headache since last week and my throat is sore , also been tired alot and stomach cramps what is the cause of this? negative for covid?

Your symptoms are consistent with a viral syndrome. Please note that home covid test can be 30-40% false negative. Whatever, the treatment is not different because you are young, only need symptomatic treatment. You can take tylenol or advil as needed for a couple days for sore throat and headache. If high fever, chest pain or dyspnea occurs you should go to ER.