hi i am 21 and have missed my period by 12 days, i know that it is common for women to miss there period but it has never happened to me before. i am having headaches, light nausea, back aches,extra tired, and cramping like my period will start but still no blood. the cramping started the day my period and is now back..i feel different like im pregnant but the test are negative so any idea what could be causing these symptoms or is there a chance i could be pregnant?