hey guys im 16 and need some advice....

my BF and i had sex 3 weeks ago
i got my period , but it was a week late , and only lasted 2 days that was dec 14/15,
i have felt so sick lately , and threw up 2 times, my lower stomach is killin me , like cramps, or like nervous feeling
i have bad headaches, my teeth are SO senstivite , never felt that way before
i am SOOO tired!!, and my lower back has never hurt this bad ,
i took a test a week ago it was neg!

someone PLEASEEE help!

thanks