Posted: 28/02/2014 at 18:05
You say "honestly nothing happened" but something changed. It may not have anything to do with running, perhaps spill-over from a completely different area of life or even a completely different time. As you say your legs are fine, but your head (also a part of your body) is not performing.
All sorts of things can affect how the brain works: food; sleep; stress from work, family, friends; it can make connections you're not consciously aware of (e.g. it's common to feel sad and not know why until you remember it's the anniversary of when X died, for example). It can make habits (of thought as well as deed) you're not aware of ...
So you need to sit down and have a good old talk with Mr. Brain like it's a grumpy teenager and ask him why he thinks he's telling you running is horrid when it's the schizzle, and where did all this come from, hey? "I'm not cross with you but this has got to stop, you know...". Maybe Brain will confess that while you were out on a run it saw someone who looked like the horrid person who bullied you at school or a PE teacher. Or something.
Best of luck anyway.