I would guess it is the dressage work. My mare used to click slightly.
She also nicked up the bulb of her heels once or twice. Higher angle, lower
angle - she still clicked. I finally started conditioning on the weekend
in sandy hill country near Brooksville, FL - where I live is pool table
flat and firm ground. After a few months of trotting and cantering up
sandy hills the clicking disapeared and never came back.
BTW when I asked my vet about the problem at the time, his reccomendations
were to not mess much with the feet but find some hills and use them.