Show clothes are definitely different than schooling clothes. I school in full seat breeches and boots, so that part didn’t need much adaption, but the show shirt, stock tie and jacket took a bit of getting use to. I am a dressage rider, but when I got my show jacket, I went with one made of the newer technical fabrics, as it has more give to it, and is a bit more comfortable. I tried a pair of knee patch breeches, but my saddles are smooth leather with no grip at all (all older saddles) with the exception of the synthetic I bought for my 3 year-old, and that has no grip either. Full seat breeches are a must for me! Kind of like riding in a car with leather seats . . . slide all over the place . . .
It could also be a bit of nerves. I have only shown once so far (second show coming up this weekend) and I did feel “different” and a bit stiffer, even though the only thing different was the show shirt and jacket. Nerves combined with the fact that you are not on your home turf and just schooling . . . all eyes are on you . . . makes us respond a bit differently.