Some people just don't put on alot of weight, hopefully you are one of the lucky ones!!!
Aslong as your dr is happy with your progress i would not worry about it!!

As for clothes i would think that eventually your clothes would not fit and you will have to wear bigger ones or maternity!!

I know that some of my clothes i can wear for my entire pregnancy. I have only put on 2.5 kg so far and i am happy with that!