Firstly, don't feel sorry for posting. There is no such thing as a stupid question or a dumb post. Everyone needs to put their worries somewhere

The scale can't just "lie" so it isn't all in your head. Weight fluctuation is normal, and it depends on what we've eaten and what time of day it is and what we are wearing that can throw the scale off. It wouldn't throw it off an entire 15 lbs, but you get the idea.

Make sure you weigh yourself at the same time every time you do weigh yourself. The best thing to do is weigh yourself first thing in the morning after you pee, and do it with no clothes on. This way you will have a more accurate weight.

Even though water retention is more common among women and varies more in women, it doesn't mean that men can't retain water. Salt intake can vary your weight as well.

You are still growing and will continue to grow into your 20s, so you probably do really need to eat more. Muscle and fat weigh differently, so if you are exercising more, then you will also see weight loss because of muscle buildup and calories burned from exercise. Fat weighs less than lean muscle.

My advice is to keep up a health exercise regimen, which will help with your anxiety issues, and to stay on a healthy diet. There is no reason to be alarmed about losing weight at this point, but you should definitely be eating more and staying away from junk food which is just empty calories, contributes to anxiety problems, and isn't going to help you stay healthy.