Mental Challenges

Saturday I had a tough run.  From the first step of the run, my head was not in it.  I convinced myself to keep running at the one mile mark...and at the 2 mile mark...and keep going to the turn around.  By the time I hit mile 3, my head was actually doing OK.  I guess I had hit my "stride" so to speak.  But then at mile 3.5 my back seized up.  I have never felt anything like that before.  It just wouldn't let me run any more.  I walked thinking that if I walked for a few strides I would be able to run again.  The walk lasted a few steps...then a half mile...then 1.5 miles.  I couldn't run any more so I had to walk back to the Bagel Shop.  What a disappointing day.  My back stayed sore and had several spasms throughout the rest of the day but it has felt fine since then.  What do you think is up with that?