Big midi problem in Cubase

in the past couple of days I’ve encountered a really big problem when recording midi in Cubase. Let’s say I’ve recorded a couple of bars of midi and then open the key editor and want to either overdub what I’ve played or add another bar.

Often what will then happen is that when I play something on the keyboard (the external or the one in the key editor) notes will start moving around in the key editor! So what was a G e.g. is now a D etc. This happens with different keyboards as well.

If anyone has a clue as to how this has started and how I can make this nightmare stop it would be extremely cool.