I Had A Mastectomy 3 Years Ago With 5 Positive

i had a mastectomy 3 years ago with 5 positive lymph nodes & been with oncologist since. had chemo & radiation & twice yearly blood work check ups. last week my tumor marker which is usually around 30 spiked to 110 & oncologist scheduling pet scan which will cost me $2000 plus out of pocket. i suggested we re-do blood work in a few weeks before proceeding to petscan. does waiting a few weeks make sense & re-doing blood work to see if tumor marker goes down?