I've just been to hospital as NHS direct thought I had a possible case of ectopic pregnancy. It happens after a few weeks from conception, for example week 7, which is what I am. You have bad abdominal pain going into the back and shoulder, often referred pain to another part of the body (which I'm experiencing) dizziness, vomiting, bleeding, cold clammy skin. Not sure what I've got but I have bad abdominal pain when I breath or move for the last three days, shoulder pain too but he thinks it's something else. Going for my EP scan anyway tomorrow so we'll know more then, fingers crossed it's trapped wind or something and not an ectopic.
What you describe is round ligament pain which is very common, in my last pregnancy I was convinced that bean was in the wrong place but actually it's them making themselves comfortable and your uterus stretching so it's a good sign your expanding down there. If you do get worried though best phone NHS they can confirm it for you quite quickly with questions.