My iPhone Display has a Spot/Smudge

Hi.. My iPhone display was perfect until recently.. but suddenly it has a small spot.. kinda liquidy smudge which when pressed becomes bigger/longer and when i apply more force it also goes up till the centre of the screen... and when I release the pressure it again goes small...

at times it is very much visible and at times its almost not there (very slightly visible) and now for a few days its Not there at all.. What could it be? Thanks.