Hi there, I do relate to the worry and the simply not knowing whether behaviour is normal or not!
Our flock sometimes freeze, head to the ground, and act perturbed. We've decided that it's to do with them hearing mice or rabbits underneath them in the field..... But of course it could be something much more sinister, or nothing at all..... Not seen them kicking their tummies though....
It'll be interesting to read what the more experienced out there say....