Most painkiller have constipation as a side effect. I believe that's because they slow down a lot of the body's functions, including the colon. That's why they can make you drowsy and make you unable to drive a car, operate machinery or impair judgments--they slow down your reactions and your reasoning.
An anti-spasmodic might work for you, or, as Lepontneuf said, a small dose of imodium. You just have to be careful that you don't slow you gut down too much; there are some people on here who took immodium for so long that they are not constanty constipated because their guts slowed down too much and won't start up again. It's kind of a no-win situation. You might go off the imodium, say, when you're going to be home and it won't matter. You know, sort of cycle on and off as you can.
You know, it wasn't until you said this that I thought about my aunt. She has Crohn's, but I was surprised to hear her say recently that she's been in remission for a number of years. My grandfather's brother's (her uncle) Crohn's was terrible, so I was surprised she had not had any bowel problems in so many years.
Now that you've mentioned pain killers, it makes sense: my aunt has been addicted to pain killers for several years now as well. I bet that's why she's not having gut problems. The only problem is she's on one that's methadone-based and she's ended up losing her teeth and she picks at her arms until they are raw and bleeding; they're all scarred up. Not to mention that when she was in a car wreck, they couldn't give her enough drugs to stop the pain; she's pratically immune to them for pain relief. Not a good trade off.