This has happened the past two times that I've ridden. Both times were within the last two weeks and the weather was similar both days: upper 50s/low 60s. At the end of the ride my horse has a little sweat on his neck (like where the breastplate would lie if he were wearing one) and plenty under the girth, but none under the saddle. Both rides consisted mainly of walking around on trails (although we did do a little stretch of cantering both rides and we did pop over some smaller fallen logs) so we're not talking about a serious workout either time. The saddle pad is cotton, and the girth is an Ovation Airform (chafeless neoprene girth). I noticed last time I rode that the underside of the saddle pad was slightly damp, so I wondered if maybe he was sweating a little bit and the pad absorbed it? The girth is neoprene so it doesn't breathe at all which would explain why he was sweaty under there.
Below are links to pictures I took last night of the saddle girthed up on his back but without the pad underneath. (Sorry if they are not fantastic. It was dark and I was by myself and he was fidgety because he wanted to go back out in the pasture and eat.) I felt around underneath it as best I could, and the panels seemed to be making good contact with his back. It also has a nice roomy channel over his spine.
The saddle was brand new in January and fit him really well at the time. (The owner of the saddle shop where I bought, Lynnda at Classic Saddlery, it is an experienced fitter and helped me fit it to him according to the wither tracing I took.) During August and September I didn't ride much because when it wasn't super hot I was helping some friends with their fence project, but a friend was still using him for the occasional beginner lesson. I am really hoping this is not a saddle fit issue!