This kind of distortion often happens when you modify the mesh that has been unwrapped. You can try to unwrap once again after modification, or ***pin*** the vertices of islands you want to stay in the same position. In *UV/Image Editor* window select all the vertices except the distorted one and press <kbd>P</kbd>.
![enter image description here][1]

Now unwrap the mesh again and unpin it using <kbd>ALT</kbd>+<kbd>P</kbd> shortcut. The modified vertices should come back to their places.
![enter image description here][2]
  


  [1]: https://i.sstatic.net/hWabD.jpg
  [2]: https://i.sstatic.net/FtCr2.jpg