I sent a Medical Examiner's certificate a few weeks ago, and now i got it back by mail, Why it was rejected?
That information can be found on the content of the letter itself.
One more for  self-certification, what if the driver's license is a commercial driver's license? Is it also possible to subscribe?
of course can also subscribe to receive email and / or text reminders about the status of their medical certifications
What does the statement about my doctor not being listed on some registry mean?
Yes, that would be the National Registry of Certified Medical Examiners Search. If your doctor is not listed there, then you must get a new exam practiced by a doctor listed on that registry.
Um, ok. It also says something about wrong information?. I was pretty sure to check that info.
Could be he made some mistake while making your exam. If that so, contact him/her so he can amend it.
I have a CDL license, which was issued before 1999. It is ok to keep it? Because i'm not part of any federal operation, by the way.
There's no problem with that. you can keep it alright. In fact,you are elegible for Excepted Intrastate. For that, you must check you license has the required restrictions.
What about if it was downgraded because medical status?