Had my 2nd pfizer vaccine shot 4 days ago, only side effect i had was arm pain day of, and fatigue next day. does this indicate i have a bad immune system based on my age? also, is there a "incubation"period for vaccine?

The reaction listed is rather common and well within the pattern of those that showed 96% effectiveness on the initial studies. It takes 2-3 weeks for any immunization to generate protective levels of antibodies with the initial dose.

No your symptoms do not indicate that you have a poor immune system. The "incubation period" is considered 2 weeks. Stay healthy and hydrated. Try to avoid contact with someone that is known to to have Covid19.

You are fine. Don't worry. We don't measure the antibody response by the severity of immediate symptoms.