Well, the blood has to be coming from somewhere, whether it's Humira or another medication or disease causing the bleeding. Just because they didn't see the source of blood on a colonoscopy doesn't mean there isn't one. In fact, I'm pretty sure there has to be one. You don't just spontaneously start leaking blood into your GI tract. This may be more detail than he's shared with you, or than you care to discuss, but do you know what color blood it was? Generally speaking, the more black it is, the further up the GI tract it originated, and the brighter red, the closer to the anus. Most of the small intestine cannot be viewed with traditional endoscopies, whether upper (via the esophagus) or lower (colonscopy). Did he have a capsule endoscopy (also known as a "pill camera") performed? That's pretty well the only way to view the small intestine from inside. Small Bowel Follow-Throughs are also useful to some degree, but don't show nearly as much detail, as it's imaging from outside the body.
The chances are not extremely high (but not miniscule either) that a child will inherit it, and low enough that I'm willing to take that gamble. My grandmother is the closest known relative with an inflammatory arthritis. She had 5 children, 9 grandchildren, and 14 great-grandchildren, and so far, I'm the only one diagnosed with it. Several cousins and both of my uncles have psoriasis which is closely linked genetically, so I expect the arthritis will pop up somewhere else eventually, but it's not as if we all have it. As I see it, I'm still thrilled to be alive despite losing the genetic lottery, so I won't assume that a child would be better off not being born just because they might inherit my diseases. And if having a child in the normal way isn't in the cards for us or we decide against it down the road, my husband and I are both very open to adoption, and my sister has also already generously offered to be a surrogate for us if I'm unable or if pregnancy would be too difficult on my already strained body.
As for what it feels like, I can't compare it to labor as I've never been in labor, but yes, I've heard other women compare it to back labor. It's individual for all of us, dependent on the severity of our disease, how much it's currently flaring, and exactly how it effects us individually, but I guess I can speak for myself. I don't actually have Ankylosing Spondylitis, but I do have the Spondylitis presentation of a related disease, Psoriatic Arthritis (they're treated with almost all of the same medications, and short of me being less likely to experience joint fusion, there are not all that many differences between the two diseases). For me, there are days where the severity of my pain leaves me unable to function, but right now, those are thankfully not the norm. Even though I am able to go to work most days and generally present myself as an average person who doesn't "look sick", and despite the slew of medications I'm on, I am still in pain to some degree every moment of every day, and have been for about the last 3 years.
That never-ending pain wears on a person in a way that someone who hasn't experienced it really cannot understand. It doesn't make you a bad person that you can't and don't understand, but it's just the reality of it. I'm trying to think of a good analogy to describe what it's like. I guess think of the pain when you stub a toe or jam a finger. It's not the end of the world, but it hurts, and it's very distracting from whatever you're trying to do at the time. Now imagine that that pain is always there, and as far as you know, will never go away. And imagine it's always in your spine, making sitting, standing, laying, and yes, sex, all inherently painful. That pain is always there distracting you from your work, your social interactions, your sexual desires, your sleep, and everything else you do and think on a daily basis. And sometimes that pain is much, much worse, making all of those activities not only difficult, but impossible. And because you don't look like there's anything wrong with you, you feel social pressure to pretend there isn't, or be taken for a hypochondriac. In short, it's hard, both psychologically and physically.
Here's another explanation that gets used a lot around the chronic disease circles on the internet. Some of it doesn't pertain directly to what it's like to have spondylitis, but a whole lot of it does. It's a good introduction to what having a chronic disease (particularly one where pain and fatigue are involved) is like. The Spoon Theory: http://butyoudontlooksick.com/navigation/BYDLS-TheSpoonTheory.pdf