Hey, I'm not really the best person to give critique, but I think that maybe you have so difficultly putting the right amount of pressure on your pen. Compared to pencil lines your brush pen lines are a bit jittery. You seem to like putting heavier pressure while drawing pencil, so when switching to brush pen, where you have to make your strokes very light must be difficult. I think maybe you should choose a different pen to line your drawings, like a ballpoint, or practice things like line control when using brush pen. But other than that, I think the fluidity in your drawings are very nice! I can almost feel the loose and tense parts of the pose :)
