If my lungs feel like they don't open up fully, what could it be?

[ncmycelium]

     I am concerned. I want to be specific about this to give as much help as possible. When I run for extended or short periods of time (.25 miles or more), I experience a very distinct feeling of my lungs simply not being able to expand enough. It's like my mind is begging them to open fully and my rib cage or something is holding them back, compressed. When I'm finished running, it takes me a long time to regain my breath, and I end up trying to open my arms, or bend weirdly just to trick myself into thinking that it will provide some sort of satisfactory breath. I am a member of the Army, so we run regularly, and I notice that I am distinctly noticeable. Others sometimes approach me and ask if I'm ok, despite my attempts to hide it. I try not to make any gasping sounds, and control my breath and keep it relaxed until I have recovered at the end of runs. Others can talk freely and have conversations while jogging, and I can barely keep it together just to keep up with them, and if they're going as fast as they like- I can't keep up. I want to be clear- I run often. I thought towards the beginning of this that it was mainly due to a sedentary lifestyle before the military, but it's now been nearly a full year of this, and my struggle to feel as if I'm breathing satisfactorily is still plaguing me. I am loath to say this is asthma because I have not felt as if my throat was tightening even once. It's purely my lungs that feel as if they are not being at all efficient and are not expanding enough, or are blocked some way. I often wonder if they are too small.

     I exercise regularly, and frequently incorporate sprinting and rapid cardiovascular exercise into my fitness plan. I can sprint rapidly for short distances, at a competitive level with my peers, but I still pay the price after a few moments, say 30 seconds of work like that. On Mondays, I run around 4 miles to the best of my ability, which usually looks like a mile at a reasonable pace, and then intense struggling to  keep moving for the rest of the period. On Tuesdays and Thursdays, I work on muscular conditioning and endurance, with sprints and jogs incorporated. On Wednesdays and Fridays, I participate in interval sprinting and jogging. To me, this seems like a reasonable recipe for improvement, especially for cardiovascular health- but I doubt this purely because while my body seems reasonably fit, my lungs cause me to struggle disproportionately.

As further evidence, when I first joined the military, I ran 2 miles in 21 minutes. In two months, I had improved that time to 2 miles in 17 minutes and some 20 seconds. 2 Months after that, I improved the time to an inconsistent time between 16:30 and 18 minutes. Some days I would run well, but most if not all were hampered by this feeling of tightness and restriction in my chest.

Does this sound like asthma, or something different? Or is it really a matter of me needing to take the time to run and work on cardiovascular fitness more- perhaps work with a trainer to choose a different style of running? Any insight would be greatly appreciated, and thank you for your time and effort.