I felt the same during my Masters. I was gulping down papers and books on stuff very new to me and churning out papers at a crazy rate. I was overwhelmed. When

I felt the same during my Masters. I was gulping down papers and books on stuff very new to me and churning out papers at a crazy rate. I was overwhelmed. When every paper came back with very high grades I actually questioned one of my professors about those grades. I literally said "I feel like I'm faking that I actually know what I'm talking about". She was amused. Looking back at it: I was working an awful lot, loved the subject, and have a way with words (at least in my native Spanish). So the output was good, what else would I expect?