Plz read this im really confused its about my period and me feeling sick at night?

ok one month is was late, the next month i missed it, and this month it only lasted for 4 days( usually it lasts 7 days). and every night i feel nauseous and my breath smells like minty puke cause i brush my teeth. i never throw up i just keep burping and my stomach hurts. so if anyone knows wat this may be please tell me.... and i took a pregnancy test and it came up negative.