My periods stopped. Is that bad?

i'm 22 yrs old, 1.55 m & 43 kgs, i havent had a period in about 7 months. ive lost a lot of weight, about 8 kgs. i'm not underweight though because my BMI is normal. i eat a lot but i exercise a lot too. i run for an hour six days a week. i also swim or cycle three days a week as well. i feel fine though, i have lots of energy, and my blood tests are good , my doctor said that i have the highest HDL cholesterol he has ever seen (2.3 mmol/L). so since im healthy, do i need to gain weight ? is it safe to not have periods for a long time? thanks for any advice :)