First of all, your doctor is right... Acoustic piano does not put out enough volume to permanently damage your hearing (unless you are amplifying it, or playing on cans with the volume too high) as a general rule. A rock concert, on the other hand...

But there are a wide range of medicines, and other things that can cause it. Here is the Wiki entry, and this may help you track down it's cause and cure, or at least, alleviation of the symptoms; http://en.wikipedia.org/wiki/Tinnitus