saddle frustration

I love my saddle. I feel secure in it, Major hasn't seemed to have a problem. A few weeks ago I had an equine body-worker out, no back problems or marks. In the last week there is now a line of white hair on his right side, right under the stirrup bar and smaller mark on his left side. I am sure the more-on-one-side shows my crookedness (which has been getting so much better). Maybe because of my increased mileage/2-pointing is is showing up? I am sure the problem has been brewing, but I haven't seen any signs of it.

I feel like a terrible mother. I am emailing with the saddle fitter, sent her some photos and will hopefully have her come out. I have been wanting to try out a treeless saddle, but really think that if I'm that heavy in the stirrups that isn't the way to go. I'm not a heavy rider, but I know that actually has little to do with it, more how you balance, etc.

arrgghhh!