Checking for pain would be what I would look at first.
And that could lead into some money too if it is something harder to diagnose.
Far to many people have a vet do a "tire kicking" on a horse, and that does nothing to answer the real question of what horse may be experiencing in terms of physical problems.
"Tire kicking" for those who aren't familiar with term is where a person just walks around wasting time while looking at car, as they aren't serious about buying it.
Used in terms of vet? Means you have not got the money to put into more expensive diagnostic testing to see what the problems are, so are basically wasting the vet's time. And not putting anyone down who can't afford major expense, but pointing out that there is no way a vet can really look at your horse, and tell you horse is fine without said tests.
X-rays, bloodwork, and the like, are needed in rule things out, as well as other tests too numerous to mention.
Hopefully it is something simple, like saddle fit, or the minor. And remember that while saddle might look fine just placed on horse, once your weight is in the seat, could be whole different matter.
Could even be the horse is not really broke to ride, or feed too hot, or your mechanics as a rider, in that your hands aren't still, or body isn't. The lunge line antics is fairly common if horse in pent up.
But could be kissing spine, ulcers, "mare syndrome" peritonitis, broken withers process or ribs, or even cracked vertebrae, or a myriad of other things.