Hard stomach?

I’m 29 weeks. I’ve noticed lately that at times my stomach will get hard out of nowhere. Sometimes it lasts a couple minutes, sometimes it’s that way for an hour. It’s not painful at all. Sometimes it’s a little uncomfortable but other times I don’t even notice unless I’m up and around or happen to touch my stomach. I was going to google it but I didn’t even know how to describe it. Is this BH or something different? Normal or no?