3 Chicks dead in the grow out pen, no signs of struggle..help please

[r4eboxer]

I just put my chicks on the ground last week they are 7 weeks old, on Sunday I found two dead with no signs of struggle, I have 4 LGD's and don't loose birds to predators. Yesterday another chick dead (my favorite too) same thing no signs of struggle. I hatched these chicks from NPIP eggs and am NPIP myself and just got tested in March. I have never had any illness in my flock. Could this be cocci? I did not start them on medicated because I picked up the wrong bag and didn't notice it until it was too late, They have gone through an entire bag of medicated feed since then but now they are dying off one by one. I see no signs of illness either, no watery eyes or nose, no bugs nothing just dead chicks. Any ideas?