Thanks Nikki - I am thinking that maybe it is fertilisation and embryo travelling as it takes up to 7-8 DPO to travel and burrow in. Something I just read said only 60% of eggies get caught for burrowing after that which is maybe why the pain stops...
I know, I know typical TWW thinking, clutching at straws... But I just can't help but get my hopes up ever so slightly.

Thanks to all of you for sharing your experiences, I am less nervous that the pain is a dangerous/bad thing now. I was terribly worried.