Thanks for posting your query,
From your description, if the bulge is retractable or reducible then the chances of it being an umbilical hernia
is high. As you described that it usually occurs when you lay on your stomach, meaning that your abdominal are highly tensed/contracted and if there is bridge on your abdominal wall, that will force intestines to bulge through the orifice hence you feeling it on your navel.
It will be best if you could pay a visit to a doctor who can take a closer look at it to determine exactly what it is.
Other possibilities are intraabdominal benign tumors.
Abdominal scan could help determine the nature of the bulge.
Hope this helps and wish you the best