Sophie,
My first thought was that your Discus was probably shedding it's intestinal lining, but without a pic or a more detailed description it's difficult to really say.
Since you've started treatment you should probably see it through, but during and after increase your WC regimen as fresh water may be the antidote.
If symptoms persist or worsen, try and get a pic, post your water parameters, and give as detailed descritpion as you can.
Jim