From the looks of it, multiple issues. The edges of the leaves are curling up too.
If feed made worse you know the NPK ratio is off, and/or it's too much for the plant. Flush/water with just water, until you see a bounce back (and look at new growth, not the already effected leaves as they will not recover). When you feed again, feed low, and make sure the values are the right balance for the plant at this stage.