It's really difficult to advise you about what your former employer might say when somebody calls asking why your employment ended. Many large employers will only verify dates of employment and last position held. Most employers will not give a potential employer medical information about a past employee. However, every employer is different and nobody could guarantee what a former employer might say to a future employer. Best of luck.
I agree your former employer will probably only give the dates of your employment, your rate of pay and job title. They may also state you resigned and/or whether you are eligible for rehire, but they are unlikely to state the reason why you resigned.