My belly button is red, sore and has a strong odour. what could be causing this?

Cellulitis: You could have a localized cellulitis of the umbilicus. Keep it clean with hydrogen peroxide and put a topical antibiotic around the area (eg. Bacitracin or triple antibiotic ointment). If the redness and smell do not go away within 7-10 days, see your doctor.

Infection.: Infection of the canal. Might want to clean this with peroxide or saline and put an antibiotic cream in there until it heals.