Lighting looks strange and planes are disappearing I'm trying to make a dolphin model for 3d printing. While making the dolphin, I realized that in object mode the tailfin's lighting looks strange:



As you can see, the lighting of the fins is split, with one side being noticeably darker. To make the tailfin I first used the knife tool on a (squished) cube to make one half of the fin. Then I reflected it to make the other side and joined them in order to connect their edges with faces. When I was done, I realized the strange lighting on the tail. I first thought that I connected some faces incorrectly, but I checked and nothing seemed wrong to me.

Another strange thing I experienced was that when I tried to slice the model on the AstroPrint slicer, some of the planes on the tailfin were missing:


What's also strange is that the pieces that were missing were the pieces part of the tailfin where the lighting was abnormal. The only part that was shown is the bottom part of the fin.

I made the dolphin's body by shaping a cylinder. I made the fins by using the knife tool on a cube and connected them with the join operation. I edited them together and joined their edges with faces. 

Does anyone know why this happens and how to fix this?
In Edit Mode with everything selected, go to Mesh menu (Blender 2.79b)/Normals/Recalculate Outside.  See if that helps.