I uv mapped a part of my character and am looking to texture paint it. I used the fill brush to fill the whole part green, but then went to the texture draw brush and then drew on the back of the character. The drawing appeared, but it flipped to the front of the mesh. I applied the mirror modifier. The image shows the uvs of the object, and the stroke that I painted on the back of the mesh, but showed up on the front.
Your normals are inverted (they are going inside the model). Texture paint paints on front normals. That's why when you think you paint the front, all is going to the back. To see the normals (edit mode) : go to the "mesh display" part of the right panel of the 3D view. To inverse the normals, select all the needed vertices, then ctrl+N or ctrl+shift+N.