I was having tightening at 16 weeks. I would look down at my tummy and see what appeared to be a ball, randomly hardening below my belly button. I asked the MW what it was and she said practice contractions- when I questioned her about getting them at 16 weeks, she told me we get them throughout pregnancy but not often do people notice them until the 3rd tri. It's only because I had no fat in my stomache that I could psychically see it happening. Apparently if I was of a 'normal' weight prior to pregnancy, I wouldn't have noticed them at all, but because I was 6 and 1/2 stone, I could basically see everything that went off down there.. Thats another reason for me feeling baby move at 15/16 weeks, and having scans that were crystal clear. APPARENTLY anyway.
But to answer your question, wobs. They feel like a tightening, and your whole tummy goes hard! If youR tummy is only hard in a certain place, say on the left hand side, but its soft everywhere else, then its the baby, but if its hard everywhere then its BH!