What is wrong with me?

[Redholm]

Im a 24 year old guy from Sweden that have been an active rockclimber for many years. I've always been in good shape and exercised regularly until i got these problems a few years ago, that only have become gradually worse.

My issue is mainly that i feel really bad when i do the slightest exercise. I can barely walk up my own stairs without feeling really short of breath, and with a skyrocketed pulse.

My hands and feet are always very red with white dots and super-heavy. it feels like im having several litres of blood in them.
I get this drowsy feeling that only becomes worse the more i walk, lift, stand, bow or do basically anything that requires me being physically active.
My resting heartrate is usually really fast (between 80-130). My oxygen levels drops under normal values every day (between 94-91%), most commonly after i've been physically active.  

I have had this issue for many years and visited alot of doctors. I have also been going through alot of investigations including, Stress EKG, Cardial ultrasound, Resting EKG and even neurological exams like MRI and other stuff. All come out clean except i was told having 36 beats per minute when i was sleeping.

Right now my Life is very miserable. I can't work and im isolating myself more and more because i feel sick all the time. The only thing i want in this world is an answer to what causes this problems.