I have to agree with Zundfolge as well, it's not the barrel length that'll make or break your conceal. It's the grip that's above the waistline. What matters is where the butt rests in relationship to your body and whether or not it presses against the fabric of your shirt. I'm not a large guy at all, being 28'' 'round the waist, and I have no problem concealing my full size XDm IWB on my hip as long as I have a coat, hoodie, or collared to cover with. A t-shirt alone will make a tell, unless it's very baggy.
If you're looking for something more concealable, go with something thinner and with a shorter grip, maybe look at the Kel-tec, Walther, or Kahr compacts. The sleekness affects your profile less, and shorter grip prevents a tell. Of course, those are little guns, an XDm 3.8 is what? A full size XDm minus an inch or so? In only one dimension?
If you already have a XDm, stick with it; or get something that actually is smaller. You'll have almost the same experience concealing the shorty as you will the full.