I don't usually get much in the way of O symptoms. But if it was positive Friday and negative today I would guess you already ovulated (probably Friday or Saturday at best guess) Kind of new to this, but I believe it turns negative after you O because your LH levels drop after the egg is released. (Mine went negative last night/today too, but I didn't test on Friday so I may have just caught the tail-end of my surge). As for the sore dry nipples, I have never personally experienced that, though my breasts do get a bit tender.
Have you been temping as well? Waiting for my temp to spike up, it dipped down today as is usually would on my O day (though I've had a few dips and rises this month haha) So that would be a good tell over the next couple days if you are.