Li Meng was experiencing an immense amount of pain. Metamorphosis, as one knows, was never an easy process to go through. The reason being is the fact that each time you undergo metamorphosis, you are breaking your body to absolutely nothing before building it back-up.
His bones were reforged multiple times due to the arts which he practised. Hell-forger art was being practiced by him till the Qi master realm. Each time a bone reforging takes place, they are pasted into a muddle and built back, strengthened. The problem with a process like that was that the next reforging was not only going to take more effort but also the pain experienced is higher.
