Sometimes, when I try to get back from the Nether to the portal in my base in the overworld, the game creates a new portal around 100 blocks away from my base. The portal in my base was active as I played in single world.
All new portals that spawned were only a few blocks away from each other.
This did not always happen, when I try to come back from the Nether, but around 1 of 3 tries.
I have had this happen once or twice. When you created your portal in your base the corresponding portal in the neather was not spawned in the correct location.
Write down the X and Z coordinates of your portal that is in your base. (viewable by hitting F3 in game)
Divide X by 8 and Z by 8.
Those numbers are the proper X and Z coordinates of the portal in the neather. My guess is that the neather portal that was automatically spawned is about 10-12 blocks away from the proper neather portal coordinates.
Best thing to do is break the natural neather portal and rebuild it in the proper coordinates.
John