It sounds like there may be an ADA (Americans with Disabilities Act) violation. Although in the earlier discussions it sounds like you brought up your health, that does not appear to be the case during the later, more problematic discussions. They should not be making such detailed inquiries about your physical health, nor scheduling a physical, without having made a firm offer of employment. And having made a firm offer of employment, they should not be revoking the offer based not upon the opinion of a doctor that you cannot perform the job duties (their doctor having apparently told them that you could) but apparently on the advice of a risk manager. That is, the offer should be conditioned only on passing the physical examination. Further, all examinations for a particular position should be identical - and it sounds like they went out of their way to make yours more rigorous than normal (even if they ultimately cancelled it).

I suggest you have your situation reviewed by an employment-side lawyer in your state who is familiar with ADA issues.