back pain and difficulty breathing while running

I am an avid runner and marathoner.  I run and/or exercise daily and other than the usual discomforts associated with such training usually don't have any major problems.  Two days ago I started a typical run and about 1 mile into it my upper/middle back started having sharp pains and I had difficulty breathing.  I continued to run and thought I would just run through it.  The pain persisted but didn't get worse.  I continued to finish the 7 mile run and as soon as I stopped the pain went away.  

The next day I ran again and the pain started almost immediately after only a few hundred feet.  I went ahead and finished my 5 mile run and the pain stopped as soon as I stopped running and wasn't breathing hard.  

I don't remember straining my back in any way, but took some Alleve because I thought it could just be inflammation, but that didn't seem to help either.

I am training for a 208 mile relay race in 6 weeks and can't afford to lay out, any suggestions to the cause of this sudden onset of pain?

Thanks for any help.