WTF?

My last session I could run maybe 15-20%.  Today, four days later, I had to scale back to 5% to keep from feeling pain.  WTF?!  Then I went to the gym and was stronger than ever.  Leg presses and leg curls went up again.  What is this stupid little muscle causing me this random issue?!  Bubba

Godsmack - "I Stand Alone" - https://www.youtube.com/watch?v=OYjZK_6i37M