Nether portal creats new nether portal

It worked out just fine until I put it to peaceful to get rid of aggressive zombie pigmen. When I then took the portal there was created a new portal under the ground (in the overworld), but the original Portal was still at the same place I had placed it. So I destroyed the new portal and tried my old portal again. It went fine to the nether, but again, I came under the ground when I went back to the overworld, but this time a little further away from the previous place. (I did this three times and all the times it created a new portal some blocks away from the previous portal, that I had destroyed.)