new nether portals being created by game

Built a nether portal, didn't find a fortress near nether side. Destroyed nether portal (diamond pickaxe) and rebuilt it elsewhere. Now when I go through the rebuilt one to the nether and come back, it creates a new portal (free obsidian!) back where the old one was located. The second time I did it, it created a second new portal immediately adjacent and perpendicular to the first new one it created.