Question about the straight punch (chokuzu) in Karate?

[OeKe]

so lately, I have noticed that when I do the straight punch for only five or ten times, i get a headache.....why? I looked in the mirror and did it and noticed that my whole body is....vibrating when I do it. Why is this happening even though I am relaxed when I thrust out my fist? is it because I'm not turning my hips when I do it?

 

[kempo_jujitsu77]

drink plenty of water. eat a small meal (toast or something) before class (not immediately before) and don't hold your breath (alot of ppl do this)