How much are you working out? And how is your nutrition? If you've been consistently working out longer and harder than your body and/or your food intake can handle, eventually you're going to hit a breaking point.
A lot of people recommend taking a week off (really, a whole week) here and there throughout the year to allow yourself to recover in a way that individual rest days just don't allow you to do. If you've been working out consistently for five or six months, this may be exactly what your body needs. You may find that you come back stronger than you were before your rest.