When you say sore, do you mean you're having muscle pain or do you mean you get saddle sores?

Are you sure you're happy with your saddle? I'm not as skinny as you (5'7", 138), but I never get issues with a sore butt. I tend to have issues with saddle sores, but that's a whole different thing.

I have some shorts with a thin chamois and one pair of shorts with a thick one, and I actually prefer the thin chamois because they have less potential to get in the way and start a saddle sore.

Have you tried different saddles or a different position on the one you already have?