Round ligament pain post-O ... What could it be??

[Serenyd]

I had a cramp in the right side of my pelvis when getting up just now, felt so much like round ligament pain. I had it after O last cycle too. I think I O'd this weekend. My OPKs are getting lighter so I doubt it's O pain. 

Any ideas what it could be?

[pokeyAC]

Bumping up for more input. 

[Serenyd]

I'm still getting it. It's weird, I don't have it at all until after I ovulate.