Distended abdomen

Hi everyone,
I'm hoping you could help me with a problem I've been dealing with since a very early age, for as long as I can remember.
The problem is : I have a distended abdomen, really distended, it makes me look pregnant. The thing is, I always had this stomach, at fist when I was young I thought (as my mum told me) that it was a baby belly and it would go away, the problem is that it never did. I'm now 25 years old and always with the distended abdomen, though I'm not really fat (I do have minor weight fluctuations, but the stomach always stuck with me even when I underweight) , I also have a very active work, and I don't drive so usually I walk a lot during the day (present situation excluded) .
Usually ( around 90% of the time) I contract my stomach, and it looks flat, I do have some fat in it, it isn't top model flat but it is flat. For obvious reasons I cannot do it when I'm sleeping which is pretty much the only time I let go, or if I'm laying on my stomach. I've also discovered that if I'm standing and I try to let go and not contract my abdomen anymore I start to feel very dizzy very quickly, as if my heart beat got slower and the air couldn't get it. As an addition to that sometimes (I wouldn't say often but not rarely neither) I experience a shooting pain in the area around my ribs everytime I breathe, and more often that not I feel like air is getting properly to my lungs, when I take a deep breath specially.
I've already told doctors about this two symptoms ( except for the distended stomach which I know I should have ) but they've concluded I was too stressed out, although it is sometimes true, it isn't always the case.This is a big issue for me, and couldn't even talk about it with no one, I'm sincerely hoping to find answers here.
I think I shared all the important details with you, thank you for reading me and for your help.