Hard uterus?

For the last week or so I thought I was experiencing braxton hicks -- I'm 16 weeks 5 days. My uterus would get hard for a little, then it would relax. I noticed it while sitting or laying down mostly, often in the middle of the night or when I need to pee or have gas. Now I'm wondering if that's what I'm experiencing or not. I have noticed that my uterus doesn't usually get hard across my abdomen, but instead usually gets hard on the right side, sometimes on the left or just in the middle. I have also noticed that this can last for several minutes. I haven't felt flutters or movements (at least that I've realized) yet. Could this hardening be baby? Or Braxton hicks?