Unfortunately when you think about things like heart rate, it affects your heart rate and makes a “loop”. You get worried about it, engage with it and the cycle continues. Maybe try doing ONE small normal activity and noticing in your head… “hey, I’m doing this activity even though my heart rate is high/low etc. Look at me go!”
I Have health ocd too. The first thing is get checked out by your doctor. I've asked my therapist about this and she said if a symptom you have persists you should get it checked out. Now if you want to start persuing another doctor because you dont trust the doctor you see then that's when you stop yourself. You really cant do anything for yourself by worrying.