“I have a change in oxygen levels after the covid vaccine?”

Hi, I have a question regarding oxygen levels. For some history, I had covid in January 2021. I am a 23 year old female. I had recently gotten the first dose of the Pfizer vaccine 2 days ago and have been monitoring my symptoms. I noticed that while I sit down my oxygen is at 99 but when I stand up and do not move my oxygen goes down to 90/85 is that normal? I only ask because my back has been hurting and sometimes it feels hard to breathe but I’m not sure if it’s just a symptom of the vaccine or if something is wrong with my oxygen levels.