Hello, i am a neurophysician Thanks for posting this question in this web site. In order to make you understand i want to stress that any given symptom can have various reasons. For example, headache can be due to simple viral fever, flu, migrain, tension headache, brain infection, brain tumor and problems related to the blood vessles of the brain. THe first objective of the attending physician is to establish the cause of headache and treat it as per the cause rather than simply giving a pain reducing medicine. Back to your question, patient with flu generall can have headache which improves over a period of time. The very fact that your headche imroves the moment you lie down and recurs when you sits up suggests the possibility of a low CSF (cerebrospinal fluid) pressure headche. This fluid is just like a protective mechanism to human brain. This is a point of concern to me and i generally advise brain scan for these types of patients to rule out the causes which can produce low CSF pressure for example a dural tear. Note that Tylenol wont help for such headache. You may try lot of water with salt to increase your blood volume. Once the cause of low CSF pressure id found out the other option is dural blood patch. Donot worry much. I strongly advise you to visit the local neurophysician and sort these issues Wish you good health