Ive experienced it quite a few times. Its not the ttokkyo tren as I have experienced the cough with both the ttokkyo and homebrew ("A's" kit). I have heard various specualtion as to why it occurs, some say its because you nicked a vein. The vein explanation seems to make some sense based on the immediacy of the reaction (cough). Anyway the cough is temporary and not something to worry about, it happens to ALOT of bro's using Tren.
One thing though I have never heard of anyone having the cough with EVERY injection usually it hit and miss. And I personally never get the cough when I inject in the quads. Weird I only get it when I inject glutes....