Is it harder to learn to fly rotary wing after learning on fixed?
Rotary flying is much harder than fixed wing to learn, however learning after you get a fixed wing license is easier in some ways because you already know:


Radio procedures
How to set up and use instruments
Navigation and flight in controlled airspace
Air law and regulations
The "gestalt" of flying


This means all you need to do is learn to fly the helicopter, which is tricky because helicopters are much less stable than fixed wing airplanes. If you take your hands off the controls of fixed wing airplane it will generally stay pointed where you put it, especially if it's trimmed properly. If you take your hands off of a helicopter's controls in hover things will go very, very badly in a real hurry so you have to be making constant control inputs. The controls of a helicopter are also very sensitive, it is a very different (and more challenging) type of flying.