Swaying/heavy feeling in lower abdomen already???

FirstTimePregnancy

I'm 18 weeks and have only had one u/s to date at 6 weeks.  We have our 20 week on January 3rd.  So far it appears that I am carrying quite low and often feel a swaying or heavy feeling in my lower abdomen when I walk.  I can imagine feeling this type of abdominal pressure/swaying when I am futher along, but so soon?  This is my first, so I am not sure if this is normal - any insight would be greatly appreciated.  Thanks!