I went into a nether portal I made with a zombie chasing me (not sure if that has anything to do with this) and it was nearly in the portal with me. When I got to the nether, I walked a couple blocks away then turned around and came back. However, when I came back, I ended up in a completely different location with a brand new portal. I didn't do anything except walk through, turn around, and come back, so I really don't know if there's anything that could have caused this. 

CORRECTION:

After going into creative to see where I was, I saw the portal was actually pretty close to the old one.