There are many normal factors that affect your energy and alertness. Even occasional exhaustion is normal.
However, does your energy take a dip around your menstrual cycle? If so, you might be experiencing fatigue due to premenstrual syndrome (PMS).
You're not alone. Many suffer from menstrual fatigue and mistake it for depression, laziness, or social withdrawal. Don't put yourself down for feeling this way.
Log any discomfort you experience throughout your menstrual cycle to understand the severity of the problem. If problems become worse, or seem to be more frequent, you should probably talk to your doctor about it.