Several things. Not knowing your age or other medical problems it is hard to say. The most likely scenario is a viral syndrome. If you feel very unwell you should speak with your doctor or do a full consult.
Many possibilities. You are best to see a doctor for evaluation then diagnosis and then treatment plan. .
Many things. As per Dr Pool. Commonest explanation is a viral illness such as "the flu". Very important though to seek urgent medical attention to rule out the rarer but more serious possibilities of meningitis or brain problem if headache is severe, unusual in character for you, if you are very drowsy or if you have neck stiffness. Difficult to comment more specifically and best to see dr for examination.