pregnancy question/concern.

I'm currently 16 wks & 4 days pregnant. This is my first pregnancy so I tend to ask questions a lot when I'm curious about things. Well lately, not everyday but some days I will notice that my lower stomach where my uterus is currently at (its still low) is sometimes bigger on one side and it feels kind of hard. Its not always like that but it happens from time to time. Any idea of what's going on?!