Four letters (you live in the US)..."F.M.L.A."
Disability law of 1993 Family Medical Leave Act, look it up if you want specifics, protects you from corrective action for absences once you're approved (Crohns is often a shoe in if documented accurately)...you have to use paid time first though for those absences (vacation, sick time etc...) and then it's unpaid, but it keeps your job. There's 2 types, intermittent and continuous...the intermittent can be taken in 15 minute increments (say you need to take 30 minutes off to clean yourself off after not making it to the bathroom quick enough! FUN!) and can be used like your situation. Continuous is for 4 days or more in a row. You would usually get approved for 480 total hours in a 12 month period of time. I got approved in Sept. 2007.
There's other elements to it too, like regulations on how they have to have your position available for you when you return, if not, an equivalent position, etc...don't quote me on that last part word for word though I read it in October and didn't get as much into those details because I know my position is in demand.
In the mean time bring a dr's note to them, have the doc or a nurse write up some vague (and non-embarrassing) info that validates your condition/symptoms. Or even bring in a packet/printout on IBD for any relevant superiors, they can read it on paper what you're living, makes it register a little better.
Like it's been said on here before, I hate the fact we look healthy on the outside and all like a bloody pulp on the inside....