Hmmm, I've personally never experienced that from jack3d, and I'm on my 4th tub. There could be several possibilities:
1. You are taking too many scoops. Remember, while some people (like me) can take 3 scoops, your body may only welcome 1.5 scoops.
2. You are not hydrating enough. If you are experiencing increased strength, stamina, and focus during workouts then it is working. Personally, I walk over to the water-fountain after every single set. Yes, every single set! It keeps me moving in between sets, instead of sitting on the machine and waiting the 45 seconds until my next set, I am moving and keeping myself hydrated.
3. I was going to recommend eating before taking it, but you said you tried that. I usually have two chicken breasts, then down a Jack3d shake 15-20 minutes after, then hit the gym 45 minutes later.
My guess is going to be hydration though. Make sure you stay hydrated, and you may need to drink something like gatorade during your workouts to keep your electrolites and carbs up.