Black Box appearing over my text, getting bigger as I type

Hello,

I am using |nDesign for my Final Major Project of my Masters. As I am typing out something, a black box seems to appear over my text, as I type more, the box appears to get bigger and bigger.

I have seen elsewhere that removing underlines from text helps resolve this, but after having done this nothing seems to have changed.

I have attached images for reference.

Any help would be appreciated as I am quite the ammateur with this software.