It could be the balance of the saddle, or it could be that the saddle itself is not sitting correctly on the horse's back (i.e. Angled too high in front or viceversa).
I used to have a Crosby dressage saddle that I loved and I could ride very well in it , but the angle of the twist in the front made my peepee mighty sore, making married life difficult. (wink wink).
Pictures and or videos please!