Burning Sensation In Chest, Breathlessness. Taking Nexium. History Of Graves Disease

For the past two weeks I ve had a burning sensation in my chest along with short rss of breath. My oxygen levels are steadily 99-100%. I feel like I am getting full breaths at times but I have random episodes where I feel like I ve just ran a marathon and I have these burning shock sensations ripple across my ribs and the SOB gets worse for a few minutes. Initially I was treated for bronchitis even tho there has been no cough or fever. After a week of antibiotics and a shot of kenalog, I was no better and saw an ENT who did a laryngoscope and diagnosed GERD . nother week has passed and I still feel the same. I ve been on Nexium 40 mg for 7 days. I quit smoking when this first started. Today is my 15th day smoke free. I have a history of Graves Disease which has been in remission since 2003. I have had anxiety and panic attacks since 1986. I am 38, 64 inches tall and weigh 118 lbs. I have had 4 children ages 15, 13, 8 and 8 months. I do not take any daily meds other than Nexium. What could be causing the SOB? It kinda feels like when I was very pregnant, but I didn t have the burning in my chest or the fiery electric shock type feelings when I was pregnant.