What Causes Shortness Of Breath And Weight Gain?

For the past few months, Ive been feeling very weird. At first, I thought I might be pregnant: irregular periods, turning feeling in my stomach, headaches, very tired. Every month Id take a test, all came back negative. Finally I went to my ob/gyn who ordered a blood test to check if I was pregnant & also check my thyroid. Thyroid came back at a 4.3, they said anything above 4.5 was abnormal so they would not prescribe me any medication. The more I looked into thyroid conditions, I really feel as if I should be medicated for this. I have shortness of breath constantly: standing for long periods of time, getting up too quick, simply walking from one room to another- all the same outcome: out of breath with rapid heartbeat as if I had just ran a mile. I recently started a new job where I have to be on my feet for 4 hours straight- how is it possible to be out of breath just standing in one spot? I have NO energy. ALSO, I have not been able to lose ONE pound since my daughter was born 13 months ago. Still to this day, I weigh exactly what I weighed at 9 months pregnant. I exercise with a stationary bike for 30mins, 4 times a week. I switched to only drinking water. I switched to ground turkey instead of ground beef. I went off Mirena birth control (which I heard causes no weight gain but just to check, I went off)... still nothing, cant lose a pound. Could this mean something other than a problem with my thyroid? Or is this CERTAINLY a thyroid problem?