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 and then unwrap. In UV/Image Editor window select all the vertices except the distorted one and press P.
Now unwrapselect the whole mesh again(A) in a 3D View window and unpinunwrap it. Being in UV/Image Editor window unpin all using ALT+P shortcut. The modified vertices should come back to their places.