Feel REALLY heavy below at 21 weeks?

Ronda 🦄🌈 • Due early April 2016 with my sweet baby girl. My first is now a little boy who was born mid December 2013.
This is my second child and I don't remember my son ever feeling this heavy so early on, only the last few weeks. 
It's like every step I take, makes the heaviness more prominent and almost hurts to walk. But it doesn't "hurt" it's just a lot of pressure at the bottom of my abdomen. 
I asked my nurse practitioner about it and she said to drink more water. Okay cool.. Helpful.. Any tips or ideas? Or does anyone know what's causing this?