Are you still in pain? Is there any blood, even spotting? Have you had an ultrasound? It could be a couple different things, it could just be your uterus stretching, or it could be something worse. If the pain has gone and not come back it's just stretching.
It just feels like it's constantly there, then goes away for a few minutes then comes back. I had an ultrasound at 5weeks 5days haven't been scheduled for another one yet. I have noticed tonight though that my lower back is hurting so bad. No bleeding though.