Doctor's excuses are a dime a dozen. Showing up with a note from a doctor that says "medical" is barely worth the paper it's written on. I could head to any walk-in clinic today, manufacture a few symptoms, and walk out with a note excusing me from work.
Yes, your employer may ask you precisely what your medical problem is and they very likely will if you are frequently absent from work. They also need to determine whether the situation falls within the FMLA or ADA. FYI - unless your situation does meet the criteria for FMLA leave, your employer has no obligation to "excuse" your absence no matter how many doctor's notes you bring in.