A bulge at the belly button is usually secondary to a hernia
, which is a hole in the muscles of the abdominal wall.
These bulges often "yo-yo" in and out, but can become "incarcerated" and permanently protrude. Less likely, an infection may be present (omphalitis): this would be warm, red, and very tender. As with most of these situations, it would be wise to see your primary care physician.