Sounds fine to me. I'm 31 weeks and I've only gained 6kg. I'd only gained a 1 or 2kg at your stage with this pregnancy and didn't gain any weight with DDs pregnancy until I was well in the 20 weeks!

Try not to worry and just keep eating healthily. You may be lucky enough to only gain a small amount. I only gained 10kg total with DD and I reckon I'll only gain a fraction more than that this time too.