Flickering pixel rainbow on the lockscreen

Hello!

I have a problem. I replaced the lcd in my iphone recently. All was smooth and flawless.. But what i started noticing now is some sort of flickering pixel rainbow which sometimes occurs in the bottom left curner of the screen. Usually it happens on the lockscreen, and disappears as soon as i slide the slider.., another place where i noticed it was the Installous app. In the download area it has a red download button, so this button flickers sometimes too..

The rest of the phone is fine.. So what might be the problem???

Maybe i bought a defective lcd?! (cost me $25) But why then this issue is not all over the phone?