One thing that will cause this is a saddle in which the stirrups are not hung correctly for that rider. I can ride one saddle and feel it pretty much, out of the saddles at the barn used for training. The others, all the same brand and style do not hurt, but the Bob Marshall is just not built right for me.
And have noticed it is the one always left on rack too, when all the others are pulled off. Out of 10 or more? That says trainers don't like it either.
Could also be your stirrups are not adjusted right, that you are not over your pelvis, or that you are "riding the saddle, not the horse".
Video? Picture of you mounted would help give a better idea.
Jeans may also be too tight, boots could be doing this? And you could need chiro for yourself too.
Horses make me a better person.