I know about mittlesmertz or cramping when you ovulate, and what that feels like for me.
But here I am at (definitely - both temps and EWCM confirm) 3DPO and I feel very mild, but fairly consistent cramping today and yesterday (but a little more today). So I hardly notice it at all during the day, but when I get a chance to sit down and rest for a moment, there it is. It's different from the usual 12 or 24 hours worth of cramping I typically get before/during O.
I've read that implantation doesn't occur until day 5 at the earliest (usually more like 7 or 8), so I know that's not it.
Edited to add - I've been sitting here a while and noticed that the right side is a little more crampy than the left. So maybe it's my ovary?? Also, the word cramp doesn't really do it justice - different from cramps during AF, and again just very very mild and dull/throbby but pretty much constant.