I think you are having anxiety
because if you asked your doctor about it, I am sure he checked your heart and lungs. You don't seem to be having panic attacks but you do seem to be having episodes of anxiety, which seems especially evidenced by the fact that they get worse when you become scared by them. You are probably carrying a lot of stress
and worrying, at least unconsciously. I would keep a record of when it happens. Anxiety can be "triggered" by anything, even a certain smell or music (or whatever) that is associated with whatever you are feeling stressed about.
When it happens try breathing in very slowly through your nose. That may help you catch your breath. Also try to fill up your belly with air, not just your chest. If this helps, you can be pretty sure it is anxiety. Perhaps it would be in your best interest to consult a therapist to find out why you are anxious if you decide that's what the problem is.