I'm shocked! I didn't think this happened until later on down the line. 
I could understand it if I had a heavy baby weighing down on the major blood vessels in my pelvis, but at 15 weeks!? (Admittedly with twins)
What I'd like to know is what is the eatiology for this so early in pregnancy? All I can find on the web is 'carrying a bit of extra water' which is a bit wishy washy and doesn't actually tell me anything. Can anyone enlighten me?