upper and mid back tightens when running, any thoughts?

Hey my names Steve and I'm 20 years old. I'm in the Navy and this morning I had a PT test so I was running my hardest. I am pretty well in shape, I usually run at least 10 miles a week and workout 4-5 times per week. Every now and then I have this problem but it was the worst it has been today. I ran pretty well the first 1/2 mile or so and then my upper and mid back tensed really tight and I could barely breathe. It felt like my back was so tight I could maybe get 1/3 to 1/2 the amount of air I normally get so this became very cumbersome. I was running pretty fast and I'm in good shape. I couldnt stop as this was a test so I had to finish it. After about another half mile of running as fast as I could with this amount of air my back lightened up so I could breathe a bit better. I then ran as fast as I could to finish it. After I finished my 1.5 mile I still could not get much air. Not like normal when I'm just out of breath, I really just couldnt open up my lungs much. I'm trying to figure out what I can do about my back tensing up. I got new shoes maybe 3 weeks ago and havent had any abnormal leg problems, also I've had this before so I dont think its my shoes. It may be possible I have a bad stride but I don't know about that either. I believe it may be that I work out my abs a lot more than my back and my back couldnt sustain my running as my abs tired out possibly? I also stretched pretty well before hand but then again im not very flexible so it could be that too. If I hadn't had this problem I feel I could have ran at least 30 seconds faster and when running 1.5 miles this is a significant difference so I really would like to solve this problem. If anything like this has happened to you or any possible solutions or thoughts would be very helpful. Thanks!