I've been feeling really insecure about my weight recently?

So I'm an 18 male senior in high school and for the last 4 years I have been perfectly fine with my weight. I've weighed a normal almost underweight BMI for my 6 foot two inches of height, and never really worried about what I ate. But lately, despite the fact that I'm a normal weight I've been feeling really insecure about my weight. I only gained like 4 pounds over Christmas break but I feel so fat. I was fat when I was a kid and I' m terrified that my metabolism is slowing down, and I'm becoming fat again. I run two-three times a week but I don't feel energized by it anymore. I've often been thinking of skipping a day without food, but never end up doing it becaused I can't think straight when I'm hungry. What should I do?