I am 26 weeks with ds3 and I have recently started to get BH's and they are really strong. They are a lot stronger than my previous pregnancies and I'm sure they've started earlier as well.
As soon as I try to do anything physical (this morning it was hoovering) they start and my bump stays rock hard and uncomfortable for about 10 minutes.
Also, did I read somewhere that being dehydrated can make them worse?