Is is possible to lose your period permanently? (i am 17, have had no periods for 1.5yrs - my periods started at 10 - i am also at a healthy bmi).

Yes, possible: Most likely you have stopped ovulating and this is the reason your periods have stopped. There are plenty of women with PCOS (polycystic ovarian syndrome) who have a normal BMI. There are also other endocrine reasons for anovulation (i.e. thyroid disease). Very rarely, some women have early ovarian failure. You should see your physician because it needs to be regulated (like with BCPs).