I'm sick, coughing mucus, runny nose. i received medicine, problem is i feel a strain on the back of my head, right side. what's wrong?

Get checked: If you are being treated for a sinus or respiratory ailment it is not uncommon to have muscle aches from the illness or from the coughing or a rough night of sleep. I would get rechecked by your doctor if you saw one to give you more answers. Good luck

Could be from cough.: It sounds like you may have strained your neck/occipital muscles from all the coughing you did. If you do not improve in a few days with massage, ibuprofen, and local compresses see a doctor for further diagnosis and treatment.