Why am I still losing weight?

I decided my weight was too low (b/c I wasn't eating enough) so I started using one of those online calorie trackers and set it for gaining 0.5 lbs per week. I've been following it fairly closely (don't always hit the target, but I'm eating more than what I'd need to keep a stable weight), but in a week I have lost 1-1.5 lbs. Even compensating for calories lost to exercise. My BMI's currently ~21.5 and I have a climber's build, so underweight/anorexia isn't an issue. Anyone have thoughts?
Why am I still losing weight?
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