BP 235/100 At The Age Of 16. What Could Be The Reason For This?

today at school i had high blood pressure . i mean apparently it was really dangerous. it was 235/100. when my teacher found out she made me lay down with my feet up and was trying to keep me relaxed and not let me stress out, i guess cause that can raise it. im 16 and underweight . my normal blood pressure is around 133/90. what was wrong with me? why would it rise so much so quickly? im goin to the doctor this weekend but im really curious right now. she continued taking it and said if it got any higher, they might call an ambulance cause i could have stroke or a heart attack . the only thing i can think of is stress and salt, that could raise, i just didnt know it could raise it that much. it really scared me though. i would just like to know causes, bc my family has healthy bp, why is mine not healthy?? also how would lying down with my feet up help? j/c