Hardness in the bottom of my belly

I'm 17 weeks today. For a week now, I've noticed that the right lower side of my stomach will be hard. The left side would be soft or vice versa. It isn't a consistent feeling. It comes and goes. I can feel it the best when I lay on my back for a few minutes. Sometimes when I walk, I'll touch my lower belly and feel it too. Is this the baby laying there or is it my uterus? Anybody else have hardness in a certain spot of your lower belly?