Nether portal won't go back to original overworld portal

I went through an overworld portal I created and arrived at the nether. When I tried to go back to my spawn (through the portal), it sent me to a random nether portal in the same overworld biome. I am almost 100% sure that portal wasn't there before. I went back to my spawn coordinates and went to test out the portal again and the same thing happened. Basically I can go into the same nether portal through my overworld portal and the one that randomly appeared. Whenever I go through the portal in the nether, it only brings me to the portal that randomly appeared. Also, I cannot destroy the random nether portal in the over world. Hopefully this can be fixed, thanks.