What happened to my photo? (background is pixelated?)

Hi everyone,

Thank you all so much for the helpful information. While a lot if it is over my head, it's very interesting to read about and I'm picking up where I can.

The photo was shot in RAW, 14-bit, imported to Lightroom (converted to DNG) and exported using  standard settings (I haven't changed much).

The wall in the background was indeed light beige, the subject was about two meters away from the wall - I'll try to keep that distance bigger as well as shoot on a black background.

I've attached another photo I took from last summer, a long exposure (30 seconds) which gave a similar result, as well as another photo from this session. Can someone help explain if I'm doing something wrong here? The photo certainly doesn't look great when viewed in Lightroom.

Thanks again.