Sharp abdominal pains?

I'm 16 weeks pregnant with twins, and lately have been getting sharp abdominal pains when I've been walking for a while. It'll be on either my right or left side, sometimes both. I know it could possibly be round ligament pain? It goes away when I sit down for a bit. I'm assuming these area just normal growing pains, but this is my first pregnancy and don't really know whats normal and what's not.