When we are working out, exercising or partaking in physical activity most of the time we push ourselves because we believe the misconception that if there is no pain there is no gain. The great news is that, it is not necessaryly so, the bad news is that most people do not know this.
In some cases it is better to stop working if we feel pain or fatigue. For one form will suffer and you will not be working the muscles you want to be and maybe your body is telling you it needs a rest.
So I say it is ok to exercise easy sometimes; drop your weight, lower the number of sets, stretch instead. Just listen to your body, it will thank you later.