I definitely had a big variation when first diagnosed. It has settled a bit now but I still have good and bad days. I used to try and work out what caused it but I've given up and just accept it now. My Dad is the same; some days he is quite sprightly despite being 90 with PD; manouvres in and out of my car, chats to strangers in the pub etc and other days really struggles with stiffness and general wellbeing.
So I think its normal! People say they are affected by stress, heat, infections, tiredness etc too. I think looking after yourself generally with good food, sleep, exercise helps maximise the good days (as it would even if you didnt have PD!) Good excuse to have a lie in sometimes and eat lots of strawberries anyway!