My sister developed asthma in her 30's. You can get adult onset asthma. I have it too and my chest will hurt so bad I think I'm having a heart issue, but it is asthma. See an allergist and/or respiratory doctor.
It sounds to me as if you are describing the onset of Chronic Obstructive Pulmonary Disease (COPD). Difficulty breathing can develop at any age, but with care, and watching your health (stopping smoking) the lungs are tremendously resilient and you may be surprised at the improvements.
Asthma can develop at any age, it is NOT just something that is found in children. That is a misconception of asthma.
I was in the hospital with what was thought to be a heart attack. Like you, all the heart tests were normal. A few weeks later, when the cardiologists were through with me, they sent me to a pulmonologist. That's when I found out I had asthma. I was in my early 50's. I had been a smoker also, but quit back in 1992.
Forums like these are quite helpful, but you need to see a doctor that can confirm your symptoms & if you do have asthma, get you on meds. Asthma can be a life-threatening disease.