trouble breathing with exercise

[spunkymcgee]

I'm a 26 yr old F, 165lbs at 5'6". I've never smoked. As a kid, I would get out of breath easily, and frequently break my sentences (even short ones) to take more breathes. I never got into sports (exept golf) or running, as I was never able to run more than 1/4 mile without getting out of breath and having to stop to walk. I did several years of breath training with debate (similar to what singers do), which helped me regulate my breathing while talking. I just figured I was out of shape (although I was 125 lbs until I got hypothyroidism 5 years ago). Since then, with my weight increase, I have tried to eat healthier and get more exercise. I started biking back and forth to school at least once a week (13 miles roundtrip). After a year of biking, I went to the grand canyon. I had no problem hiking down for a couple of hours, but on the way up, I had to stop every 20 paces (literally) and catch my breathe. I even started hyperventilating a couple of times when I tried to push myself. I was gasping air in and out of my lungs, but it didn't feel like I was getting any air. It stopped after about 10 gasps.

The last year, I've been training for a half marathon. When I started, I could only jog 1/4 mile at a time without stopping to walk, although I could alternate jogging and walking 1/4 mile stretches for about 6-7 miles without even getting sore the next day. I thought I was out of shape and should push myself to jog longer stretches. I got to the point where I could control my breathing long enough to jog a mile straight without stopping, but I would get a blood taste in my mouth after doing so. I read and heard that this taste was just lactic acid and wasn't harmful; it just meant I was pushing myself. So, I continued my training, jogging (8 minute jog, then 5 walk, 5 jog for 2-3 miles), walking 6-7 miles, and walking 9-13 miles every week.

I just completed the half marathon. I started out well, 4 minutes walking, 8 jogging, then 4 walking, 4 jogging, 5W, 4J, 6W, 3J, 6W, 2J, 6W, 2J. That completed about 4 miles. Then there was a strong uphill gradient for a mile. I walked the whole mile, and was out of breath at the top. I felt like my chest was slightly restricted, but it wasn't bad. I went downhill another mile (mainly walking, a 2minute jog), and then uphill another half mile (all walking). At that time, my lungs felt like they were encased in spandex; I could barely force them to expand. They felt like sore muscles; I intensely didn't want to continue breathing. I kept walking, a couple of miles downhill and flat. Then, I started putting in 1-2 minute jogs into the mix. Once, I jogged longer than I should have, and I gasped (almost hyperventilating). So, I took it easy until the very end. Once I saw the finish line, I started running hard, and found that I was gasping for air. I had to stop running, as I was hyperventilating, exactly as I had done at the grand canyon.  

I don't know what's going on. I always thought I was just out of shape. I can walk 13 miles without getting sore; I can bike 13 miles (at a 10mph pace) without any trouble. If I climb a flight of stairs, I get out of breath. If I jog too hard or too far, I hyperventilate. My breathing gets better with practice; I can control my breathing longer than I used to be able to. But, it just doesn't seem normal.

Am I just out of shape? Could there be something else that is causing my respiratory problems? Should I keep training? Should I see a doctor?