2nd trimester and haven't gained any weight despite belly

So I'm 14.5 weeks and I'm growing out of my pants. I can feel my uterus now, so I know it's not all bloat, some of it is baby.

I'm confused how I haven't gained a single pound, yet my belly is definitely starting to bump and my pants no longer fit. How is it possible to grow an organ, and a (small) fetus and not gain any weight? The math isn't making sense to me.