Am I undereating/not eating enough?

(Note: I am not trying to starve myself at all)
I am wondering if I am underweight because I am a picky eater and my parents don't really buy the foods I like anymore so today I only ate a hamburger (which I made by myself), Greek yogurt, and a grilled cheese sandwich which approximately adds up to around 400-500 calories. I have been eating like this for weeks and I've already lost a few pounds and that makes me happy though. So I am I undereating?
Am I undereating/not eating enough?
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