Unstable menstrual cycle. need help!!?

I am 17. My big exams around d corner. im counting tru dys with stressful and pressure. i usually have stable menstrual cycle which i dont have to worry muc but lately i realize my period hasnt come out since last month. im worried because i may got pregnant so i have testd for pregnancy test, it gives negative result. my bf give me pills for periods (norelut) but it doesnt work. with all the exams pressure and this unstable period, what should i do?