Add New Layer is putting layers out of order.

I'm sure I've accidentally hit some shortcut or something. Just recently the "add new layer" is adding layers beneath the selected layer instead of on top. So instead of having the numbers go from the bottom up, 1 2 3 4 5, etc..., they go top down 12345 etc...Why? New layers have always been on top of the selected layer, or at the top of the layers panel, but now they're being added downward, beneath one another, if that makes sense?