Weight Loss at 33 Weeks?

I am currently 33 weeks. I had my appointment this morning, and it turns out I lost a couple pounds from my last appointment two weeks ago. 
That means I've gained a total of only 8.5 this whole pregnancy. I was sure I had gained weight, as I feel my belly has grown quite a bit recently. I was average weight to start, and my doctor doesn't seem concerned. Anyone else been in a similar boat? I'm just a bit concerned I guess. Thanks!