Is it illegal for an employee to disclose medical information to a prospective employer?

Hello everyone. First and foremost, I have no intentions in suing anyone. The reason for this is because I did, indeed get the job. Now let me explain. I had a job for about a year until the company basically went under due to the economic situation we're in right now. I was the last one to leave the job because my role at the job was so important and was still doing work for them from home. They were very aware that I was looking for another position. Actually, they told me I should. It was pretty hard trying to live off working about 10 hours a month from home doing web development work. I finally found a great, full-time position. This particular employer had all the intentions to hire me after the interview, but just wanted to check my references. So he called my previous employer. He never told me what was said, but told me that my previous employer, for the most part, had nothing but good things to say about me. Therefore, I was hired. One month into the job I got a little sick (I have a chronic illness, which can cause excruciating pain at anytime in any part of the body). I did not miss any work, but I wanted to inform my employer of my situation. He was actually pretty cool with it. He was just concerned, and I told him I was fine and felt he needed to know early so he's not left in the dark if I call out one day because I'm in the hospital. He then told me that when he called my previous employer about the reference she told him that I had an illness. And that when she started to tell him this she said she didn't want to go any further because she didn't want to get into trouble. First of all, my illness never effected my performance at that job. When I was hospitalized I did my best to try to get work done from the hospital bed. I would stay up late nights to try to make up work. They always made it seem like it's OK, and that your health is more important than work. Now she make it seems like it was a problem when I'm getting another job. My current employer told me that what she said did not effect his decision in hiring me one bit. I'm just happy I don't have a @$$hole for an employer or I could have lost a job opportunity. Is this illegal what my previous employer did?