Perfectly normal. You will have to gain around 10 kg by the end of your term. So the weight increase should necessarily happen after 20 weeks. Before which its totally fine. Your baby will take all from ur body. Dont skip the prenatals though.
Don't worry too much about it.
I've lost considerable weight with each of my pregnancies.
As long as your doctor is keeping a close eye on you both, and your little bean is growing on schedule, there's nothing to worry about.