It can also be because the cat doesn't understand what is going on. It might think you are torturing some poor animal with the hums and sings that come from the instrument. No insult to your playing, I think animals in general don't get music.
I can share a story of how I got my cat to calm down and accept the vacuum cleaner. She was deathly afraid / hated of it, no surprise really, but the hate went on after I stopped vacuuming. She would go over and pick a fight with the thing, swatting it for a reaction. Kind of entertaining really.
I would go over and calm the cat, touching the hose while petting her, moving the hose a little bit. It kind of worked, but then the next cleaning session brought the fear back. I tried the same petting while touching the cleaner while it was on, over time it worked. I could eventually put the machine on low speed and could even get to the point of vacuuming her!
She never fully made peace with the machine, but learned that the massaging suction feeling was actually quite nice. And as long as I was at the same side of the machine as her, she allowed it. I hope nobody misunderstands and thinks I tortured my cat, this was all done using affection as encouragement. No force.
The point I want to make is that you can reassure your cat that nothing bad is going on. Try playing a few notes and let her curiosity come over, hopefully the cat will investigate, smell around and find that there's really nothing crazy going on.