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.
Truman
Truman Prevatt
Sarasota, FL