I think it's normal. When you start work, you feel you are expected to know nothing about the project, so you don't feel so bad.
But after a few weeks, you feel frustrated because it dawns that to perform incrementally more in your job, you will need to
learn a huge amount. It is like writer's block, and always happens a few weeks into a job. (at least for me).
The solution is to always think of the bigger picture - the code is just one portion of the entire design, and the design is just one
portion of the product and the product is just one portion of the solution etc, and the solution is one portion of the
whole story that will get to the point that money is exchanged and the company makes revenue.
So, don't forget the overall goal and you'll naturally allocate appropriate time in your head to different bits of the design,
and have a clearer idea of the bottlenecks to focus on, maybe a better idea than the project manager.. then that will
make you know which bits of the design to learn and focus on, to get your portions complete.
Of course, this is just a personal theory.. everyone works differently.