If you're thin and have no belly your cumberbun has nothing to latch under.
This means that most stock BC's will ride up on you, because even in a horizontal position, you're shoulders are tilted higher and the air likewise will ride up the BC and ride up on you.
Only solution is to get a crotch strap. There's no option for that with the Axiom I3 unless you do it yourself.
Tightening the shoulder straps does NOT tighten the BC contrary to popular belief. All it does it raise the pocket position and limit how high the BC can ride up. But then you can't access your pockets easily unless you're were born a Sea Otter with their awkwardly adorable armpit pouches.
Here was my solution before I decided to learn how to dive a Bp/W.
It works and it's pretty secure to.
I used some neoprene scrap fabric and glued a triglide with Neoprene Cement.
I was then able to thread a crotch strap, once that connection cured.
You could use Webbing and AquaSeal as well.
I can give you more specific directions if interested.
With a crotch strap however you add new hazards to diving, specifically weightbelt placement under or over the crotch strap. There are advantages and disadvantages to both.