Monitor suddenly affected by magnetic fields?

I don’t know much about monitors but I have an LG 17’ flat.

I have always had my monitor and pc case close together on the table and everything’s been fine.
But last weekend I went to a lan party and when I came home I noticed that the right side of the screen (towards my pc case) turned greenish. :-/ The closer I put it to the pc case the greener it got.
I usually don’t have the left-side cover of the case on but I tried putting it on and it made things even worse.

So how come my monitor suddenly got so sensitive about these things?:huh: