Thanks, folks! Well, it doesn't seem to be a problem with dressage lessons (sitting trot including), and I definitely don't arch the back (or the instructor (both of them actually) would go after me). So I wonder if it may be a shock? I ride lesson horse, so I don't have my usual Thinline pad (I always use with my own mares). May be I have to give it a try with Thinline (the only problem I'm afraid to forget it on horse and it's not cheap to lose :) ).
I didn't think about the chiro, but I wonder if it'll be paid by my insurance if I just show up there complaining.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."