post birth control help?

I'm 16 years old and I've been 5'4 since the sixth grade. Throughout middle school and freshman year, I weighed between 128 and 132 pounds. Sophomore year, I went on birth control and weighed about 140.

I had complications with birth control and I decided to go off of it in March, but now I'm losing unwanted weight. I usually wouldn't care about the number but it's the fact that within five months, I've dropped eighteen pounds and I haven't had any change in diet or physical activity.

I know I'm sixteen and still technically not 100% developed yet, but eighteen pounds is a lot and I don't want my weight to keep dropping. Is this normal after birth control?