I'm on my third pregnancy and this is the first time I've noticed them before 38wks. I started to notice at around 22wks or so this time. I would think you are feeling your uterus stretching rather than braxton hicks, its a kinda similar feeling...but maybe some folk can feel them that early.
I'm 27+2 , and just realized I've been having them. To me it feels as tho my abdomen is stretching or tightening. And I get minor lower back pain. I just thought it was baby moving around at first. I'm not sure tho how early they start, or how early is too early! sorry )-: