I have noticed there is cracking around one window on the landing, I recently chiselled a groove along the cracked plaster and noticed that the bricks beneath have cracked. I have filled the crack with plaster and it was fine for several months and now there is a thin crack appearing again. this is an external wall and there are no cracks on the outside of the property just the inside, the crack runs diagonal from the window to the floor, should I be worried?