Since my teens I've had many times when I've felt like that and as others have said it's usually tiredness, stress or even depression. I've learned to deal with it by discovering it's not uncommon and I'm not going mad! :) Distraction can help - if possible try not to dwell on the way you feel and do something that occupies your mind, watch trivia tv, read a magazine, go outdoors for a walk/gardening. Try not to worry or you end up in a downward spiral...if it starts to really bother you see your gp.