Hi, I'm A 25 Year Old Female, Normal Weight, Normal

Hi, I'm a 25 year old female, normal weight, normal height, diagnosed with JRA at 12 years old. I'm currently in remission and not on RA medications. I am however having a little bit of sore knees once again. Plus, some other odd symptoms that are concerning me. I've recently used my grandparents Oxometer (spelling) after some light exercise, and found my O2 running below 85 and even as low as the 70's. My pulse also is showing 120 and higher. When I rest for a few minutes, it all goes back to normal. I've checked various symptom checkers online and have a call into my doctor. My aunt said I should go to the ER, but I would rather wait for a normal Dr. appointment. I am just extremely curious and concerned and want to know what could possibly cause these symptoms?