What Causes Shortness Of Breath Even For Simple Tasks?

Hello,
Within the past few months, I noticed that I've been having more difficult breathing as a result of simpler activities, such as going up the stairs and moving heavy objects. I am 24 years old, 5'2" and 125 lbs. 5 years ago, I came to Colorado from Florida and weighed 180 lbs, yet I seemed to be in better shape than I am now. My primary care tested me for anemia and blood clots and had a chest X-ray done. She then sent me to a pulmonologist, who gave me a full PFT, echo, and methacholine challenge. Everything was normal, but the pulmonologist noticed that I had consistently high pulse rates (around 98 on both visits). The first visit, he had me go up a flight of stairs and my pulse went to 125. On the second visit, he was about to send me home and tell me I was just out of shape and need to exercise, but sent me to a cardiologist after he checked my pulse (after sitting in the waiting room for 40 minutes, it was 98). My resting pulsing rate used to be 70-75 but has since gone to 80-105. It typically raises dramatically when I do activity (like 135 just walking 15 feet), but these high readings aren't consistent. Some days I can go up the stairs and my pulse goes up to 110. Other days, it'll go up to 160. Is it really possible that I'm that out of shape at my age AND some days are better than others? I'm not super active but at the minimum, I'm on my feet running around at work 8 hours a day 2-3 days a week, which is more activity than a friend around my age. My cardiologist seems to think that I might have Wolff Parkinson White because of a tiny blip on the ECG that doesn't look nearly as prominent as the typical ECG reading for Wolff Parkinson. Are there any other ideas of possible causes?