It sounds like you're really not feeling good at all! It's possible that you're having an extra-bad premenstrual week or that you're dealing with an illness but to get down to the bottom of things you need to take a pregnancy test first to see if that's the cause of your symptoms.
Since your period isn't due for four more days, it's possible that it's too early yet to take a pregnancy test. However, since your symptoms are so pronounced, I think there's a good chance you'll get an accurate result either way. So get an early result pregnancy test (one that can tell you before your missed period) and take that. If it's negative and your symptoms continue past the day you're supposed to start your period you should call your doctor.