weight loss help

I'm EBF and I'm naturally quite small. My pregnancy bump was small but my baby was huge for my size (8 pounds 9). At 3 weeks PP I could fit into my clothes. 

My baby is now 17 weeks and weighs 16 pounds 8. Apart from the fact that my back is broke lugging him around, I'm also struggling with my weight. 

He eats constantly. He eats plenty during the day and last week moved from 1 night feed to eating every 1 1/2-2 hours at night (which is also killing me). 

I'm eating plenty too, more than I ever did before I got pregnant. I can't keep up with him though and I'm losing weight pretty constantly. I now weigh about 8 pounds less than I did before I got pregnant,but not in a good way. I'm far too thin and look ill. I don't know what to do about it. I do still want to feed my baby. I also don't feel this is something I can talk to with anyone in "real" life because it's seen as not a problem. People just say "I wish I had that problem ". In baby groups I can't mention it because the other girls are struggling with weight in a different way and none of the baby books even mention that losing too much weight even happens. 

I'd really love some advice.