Braxton Hicks or not?

I've only recently read up on Braxton hicks and not sure that that's what I'm experiencing because I'm only 17 weeks. Every now and then I feel my uterus get rock hard (about 2cm below my belly button) last for a minute and it's not painful but rather sensitive. I feel it a lot of I lay on my back but sometimes it comes spontaneously while walking, driving, sitting etc. This normal?