Hard to say without looking at your position on the bike. Are you wearing padded cycling shorts? Those might help. You should not be sitting directly on your tailbone. You should be leaning slightly forward.

When you do jumps, are you coming down hard on the saddle?

Make sure that your power is coming from your gluteals. Know that little part where the butt kinda sags down? Yeah, right there. That should cause you to lean forward.

That's all I can think of. There are spinning instructors on this board, however, who can help.