I have had two kinds of non-labour contractions in pregnancy, and I have never been sure which (if either) counts as BH. Most commonly I have a strong tightening of my uterus, not painful but uncomfortable as it feels like there is a bowling ball in my abdomen pressing down on everything! I have had this from around 16 weeks onwards with this pregnancy and my first, it is usually worse with activity or if my bladder is full. Then more recently, just with this pregnancy I think, I have what feels a bit more like you are describing - period-type cramps which tend to last for only a few seconds, a couple of times longer. I don't think I've experienced the much longer cramps you have, although I do have periodic aches and pains in my lower abdominal area which I presumed was just stretching etc.
I always thought BH were meant to be fairly short in duration, although it sounds like whatever you are experiencing it is your body gearing up for labour all right!