What Causes Belly Button To Disappear?

My belly button has been slowly disappearing over last few years. I am 63 and have been normal to thin in weight for last 10 or more years. Never was obese either. Have had one child. No pain, no symptoms, no lumps or swellings. Have had no surgeries other than two internal scrapings of uterus 30 years ago for dysmenorrhea. Even my little finger no longer fits in. I've asked a few doctors and they just look perplexed. Ultrasound shows nothing unusual, nor does gyn or GI exam. Can anyone help? Thanks.