Not pregnant. It cud b tension headache. In case of any doubt u can simply do Urine pregnancy test when your period r missed. u r v.young. having sex at earlier age n with different partners increases the risk of sexually transmitted diseases/pelvic inflammatory diseases/hiv n hpv infections/cervix cancer n other problems in later life.use proper contraception n indulge in safe sex practices. N better get vaccinated against HPV for primary prevention of cancer cervix n other beningn vaginal n anal diseases