Hello,
I cannot confirm anything without examination but it can be due to yeast infection of the belly button or due to some other fungal infection. Fungal infections often present with bad smell
bud gently clean the navel. Also apply some mild steroid cream like dermacort and antifungal cream like clotrimazole and see if it helps. If the symptoms persist then oral antifungal drugs may be needed. Please consult a dermatologist in that case. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.