It could be a migraine. From your symptoms that's what it sounds like, at least. I know because that used to happen to me all the time. Anyway, it could be any number of things, but a most likely answer would be that you are stressed. Relaxation is key if that is your problem. Whereas, it could also be the fact that you may be straining as you do something, such as, straining your eyes while reading. If that's the case, you may want to get your eyes checked and really try to relax your face more as you do your daily work or whatever it is you spend your time doing.