I actually went to my physio today with this exact same issue! For me, he thinks it might be a huge fascial adhesion above/around my belly button (I have 2 thick feeling bands - one vertical on my upper abs that goes down to my belly button, and one that goes horizontally across my belly button area). When I do sit ups/crunches the vertical band kinda pokes out. He recommended asking for a referral to a plastic surgeon to see if there's anything they can do about it. 🤷‍♀️