16 Year Old With Distended Abdomen. What Is The Cause And Treatment?

Hi,
Hope you are doing good. I started working out when I was 16 years old (I am now 18), so far it's going perfectly well except for one little thing. My abdomen is swollen, all the time. My BMI is 7.8% so it's not fat that causes it, I can see abs but it is swollen. It's like if I was pregnant or something, the lower it is, the swollen it gets (especially in the pubis area). I thought it would go away if I kept training but it stayed. I then thought it might be that way because of constipation, but I'm never constipated and am going to the bathroom 2 times a day. Then I read somewhere it could be retention, but I don't eat pre-made meals and i never add salt and the few times I do it is very minimal. I eat fruits, vegetables and nuts. I also thought it might be wheat so I stopped eating food made out of it (pasta, bread, muffins ect...) and replaced them with spelt or corn flour which is the only thing that showed a tiny-bit-small change (really not that much) and ate no wheat for 3 months or so but nothing really changed. I'm thinking maybe it's because of the small the distance between my ribs and my pelvis (which is 3 inches). I amwondering; Is there anything, medically speaking, that could cause that? Or is there any medical treatments, or whatever, that could "fix" my "problem"? Thanks a lot in advance and sorry for the mistakes if there are any (english is not my first language) Good day!