I've been gaining weight on a low carb (w veggies) high fat n protein diet (alot of nuts) but my periods r nt cmg bk? Lost it @ bmi of 18.8. Now 18.2.

Amenorrhea. Being underweight is known cause of amenorrhea or lack of menses. Normal body mass index is 18.5-24.9. If you "lost" your menses at BMI 18.8 and now have BMI 18.2, then you're losing weight, not gaining, assuming your height hasn't increased. You might have to gain even more weight before you start menstruating regularly again. This also assumes that you haven't engaged in unprotected sex.