Reply posted for akahige.
First, keep in mind that an employer only has to make reasonable accommodations, and reasonable does not mean the same thing to everyone.
Regarding you making available your medical records, I am not sure if the employer has a right to demand your records just to determine if the ADA applies to your situation (which is does apply for Crohn's and colitis). Employers are usually accepting of just getting a note/explanation from the doctor. I don't even understand how the employer would go about determining your medical condition based on your records since presumably there is a lack of medical knowledge.
I know not much help, but I would suggest either contacting an attorney or the ADA info line. If you go to an attorney, it should be an employment law attorney (NOT a disability attorney - they handle disability claims and might not be familiar with how the ADA works). I am not sure where you are located, but most local and/or state bar associations have programs available for reduced rate consultations if an attorney does not already provide free consultations.
The other option is to try the ADA info line (800-514-0301). I am not sure if they can answer whether you are required to provide your medical file to the employer or not, but it might be worth a try to ask. You can also try the ADA website to see if there is any information already available to employees with disabilities.
Good luck and let me know what happens.