my ob mentioned BH to me very early on - basically said every time you move from being in one spot, your uterus contracts - even when not pg - you just don't notice it when not pg, or very early in pg. when you get larger from pg, and move suddenly from being in same position for ages (as you do when you wake up to go to the loo at night) the BH will be more noticable. it's possible that first movement starts them cos your body is relaxed, and then they keep going

i'm noticing tightenings when i get up from sitting for too long, or when i go to the loo in the middle of the night - thankfully, at this point they're not painful - but definitely there - and it takes a while to get comfy again afterwards...