I Have Been Increasingly Experiencing SOB. It Feels Like The

I have been increasingly experiencing SOB. It feels like the air is not getting past the back of my throat. I used to be very active walking, interval running, strength training. I am not able to do it anymore. My O2 sats are 96%. Just had a physical and Dr doesn't know what could be causing this. I am not an axious person but this is starting to worry me. What could I look at?