Essentially the INFLUENZA virus is very vicious and can cause intense problems. If you just got the "stomach flu" that actually has NOTHING to do with the flu shot, it's not even the flu. The flu shot itself is supposed to be the flu, except dead or empty shells, which tricks your immune system into seeing it and thinking it's the real thing, so it can figure out what to look for should you actually get Influenza and react quicker.
Essentially it IS possible to become infected by having dormant or only half dead virus cells, but the chances are slim. Commonly though, people just refer to the flu as stomach pains and throwing up, and thusly get that after getting a flu shot then think it didn't work. Well the "flu shot" did, you just got somethin else.
All in all, as #15 said, the flu shot did nothing to cause this, just coincidental timing.
actually, the flu shot contains weakened strains of 3 different types of the virus, so either you caught one that wasn't contained in the flu shot, or if you're allergic to eggs it could have triggered a reaction.
im in nursing school and we did a free shot clinic, so i figured what the heck? i'll get one too... first time ever, usually against it because i never get sick... guess what? i got sick. it happens more than you think.