Gasping for air/chest discomfort

27 year old female. Only meds is NuvaRing. No medical history but migraines (over a year since last one). Active, recently began training for marathon.
Approximately 5 weeks ago I started to have SOB. Like gasping for air and not being able to take deep breaths. Mild chest pressure/tightness. Next day I went to a doctor and got a Z-pack, medrol dose pack, and albuterol inhaler. O2 stats are normal. 3 days later, no improvement, and worsening. Went to another PCP, had EKG with no significant findings. Next day had a CXR, pulmonary CTA, cbc/bmp/thyroid labs, and echo. (Radiologist is known to be a poor one). Labs normal. 3 weeks later, PCP office called stating I had borderline RVH on echo. Went to a cardiologist that day, redid the echo and stated normal. Wore a holter monitor the next day for 24 hours (no results yet, but assuming normal since I never got a call). I have a stress test scheduled for next week. However, it is worsening. I am no longer able to lie on my back. Walking down the hall (100 ft) causes me to stop, bend over and lean my shoulders in to get a gasp of air. It’s a constant feeling of someone sitting on my chest now. It feels like bra is too tight at times due to pressure and catch myself pulling it off chest. I do not snore, I never have. By the end of the day, I am exhausted. I gave up caffeine for two days, no difference. I called off a day of work to relax (stressful job but nothing new to make it more stressful than years ago), but no change. Running out of ideas…