What does early contractions feel like????

FTM here, and I have no idea how contractions are going to feel when it starts.. I'm almost 40 weeks and haven't experienced Braxton hicks (that I'm aware of)... but for almost an hour now my stomach hardens mostly the top of my stomach every 10-12 min, it's been regular so far.. no pain or cramping, just my stomach feels like it turns into a rock.. is this Braxton hicks, or the start of something else??? 😳