I have had a belly button piercing for at least 5 years and recently, the inside of my belly button (not the piercing hole) is bright red.

It is possible that the redness in your navel is due to a bacterial infection or a fungal skin infection. You can try cleaning it thoroughly with mild soap and water during showers or baths, drying it gently with a soft towel or Q-tips, and avoiding antibiotic ointments unless prescribed by a doctor. If you do not see improvement within 3 to 4 days, it would be wise to see a doctor for a proper diagnosis and treatment. If you experience clear or colored discharge, foul odors, increasing pain/redness, fever symptoms, or intense itching, seek medical attention promptly.