Low weight gain

I'm currently 21w3d pregnant and I've only gained half a pound. I asked my ob if this was okay (his records say I've gained almost 2). He asked if I was eating, and I said of course. So he said I'm fine and soon enough I'll be complaining about my weight gain.   It was reassuring to hear him say that, but it's still weird to me such little weight gain. Is anyone else experiencing this? Thanks!