I can see that she's clamping her tail, and it looks as though the fin is burned at the tail edge. This could be from a "burn" from ammonia or nitrite or it could be a bacterial (or possibly fungal) infection.
It could be the angle of the photo, but there looks to be a film on the water surface as well. Have you tested your ammonia and nitrites? If either is not zero, you need to do a water change on the order of 50%, which I would do anyway. (Water changes are almost always the best medicine in my opinion.) Please advise with the ammonia and nitrite readings, and then we'll figure out hte next step.