Retroperitoneum is a region of the body that lies behind the gut. Obscured by bowel gas means that the retroperitoneum could not be seen clearly as your gut might be having a little bit of gas at the time of the ultrasound.
It doesn't mean that anything is wrong.
Next Steps
Kindly consult a general surgeon for physical examination.
Health Tips
Get a repeat ultrasound when your abdomen feels light.