You are riding dressage, right? I mean that is a dressage saddle, isn't it? Have you trained before in a hunt seat? The way you have your calf against the hrose's side with your toe out looks like a jumping seat. Your stirrups are a bit too short, maybe, or it could be that the saddle is simply not a good fit for you. See where your knee points along the curve of the flap? It should point at the farthest out part of the curve, but yours is pointing above that, as if the saddle is too big for you, or your stiruups too short. The angle behind your knee is a 90 degree angle, which is about right for a novice level dressage rider, though.
It looks as if that saddle may also not fit the horse so well, either. It seems like it might be down on top of his wither bone. He is really hollowing out his back. Have you looked at the fit of the saddle on him?
In many of those pics you are putting a hollow into YOUR back, too. Mayvbe that's overcompensation from hunching before? But you might like to tuck your tail bone under you a tad more, to make your seatbones point straight down, instead of pointing out behind you as it appears.
This is the best photo, IMO.