The tension is there, but you may not be able to feel it yet.  It can be quite subtle. Later you will start to recognize it, so just Release and Relax, even if you don’t really feel something. 

 Since your attention was drawn away from the meditation, you can be sure there was actually tension there. If there was no tension, you would not have been distracted.

6R it once and Return to the object of meditation.

 You aren’t going to eliminate the headache or tension in one go, but you can Release your attention from it.

If attention keeps returning to that tension, you are basically feeding it, making it stay or even get worse. If you leave it alone, the tension will diminish on its own. Understand that this is just a painful feeling. Use the 6R’s, but only whenever your attention gets drawn to that pain again.