I'm weight-painting a dog mesh, but the "Add" brush is subtracting from the spot where I click, and instead adding to the corresponding mirrored part on the opposite side of the mesh, over the Y axis. The problem becomes worse as I approach the crease between the selected leg bone and the underbelly, and doesn't seem to be a problem when I am painting the leg itself.
Weight paint was working fine yesterday, and since then I have made edits to the armature and reapplied the "symmetrize" function more than once. I haven't made any changes to the painting options except to make my brush stronger.