Nerves
I too can suffer from slight nerves just before an LPC/OPC - I think if truth were to be told most pilots suffer from slight nerves even though they may not admit to it!
For me the key is not too over analyse what you about to do and just go with flow.
Once I'm sitting in my seat (important get good seat position and rudder pedals adjusted for you position) and called for the first checklist I then go with the flow and what's ever thrown at me deal with it accordingly (which sometimes is not pretty!)