Help ! whats wrong with me!?

ok so friday i tried on a pair of old that were lying in my closet jeans and they were supper tight on me and wouldn't on, Saturday i tried them on again and they fit perfectly..? then today i wearied my favorit jeans that i wearied on monday and they fit perfectly and today they were baggy on me? and a sweat **** i bought last week that fit perfect and i wearied it on friday and it was baggy? im not trying to loss weight im 15 and im only 100lbs so i would i want to do that !? its not just my sudden weight loss i worried about .. i keep getting REALLY thirty and iv been downing water bottles in like one sip! and iv been really sleepy this past week. help ! is something wrong with me !!?