maybe uve eating too close to the workout so it havent started digested yet.
before i used to eat just 20 mins or so before workout and i sometimes thruw up in the gym, i started to take a little more time between meals and workouts and it stooped, and i was able to go to failure again whitout getting these feelings.
can also be that u drink to little water during the day or during the workout.
if u have something in ur mind, and u think that it causing it, avoid it for a couple days.
ussully it is.