I have this if I don't eat properly as has been said - for me adding iodine in the form of kelp every day really helped with my energy levels overall.
However, for a while I had this excessive tiredness certain times of the month. I could just lay down and go to sleep anywhere. Turned out it was connected to progesterone levels - this can be a problem for some PCOSers - are your hormone levels all checking out okay? Sometimes though I think we are more sensitive to slight fluctuations in hormone levels as well! Here's some info I found
Foods to Raise Progesterone Levels
Turmeric, found in curry, is known to help increase the body's progesterone levels. Other herbs such as thyme and oregano are thought to have the same properties. Adding one of these ingredients in each meal may help to maintain hormonal balance.
Foods rich in vitamin B-6 including walnuts, whole grains, fortified cereals, and soy milk can also help maintain estrogen-progesterone balance. Also, try to only eat hormone-free meats, poultry and dairy which can disrupt hormonal balance.
Natural Remedies for Increasing Progesterone
More and more women are opting for a more natural approach to increase progesterone levels. Because synthetic progestins are not exact replicas of the body’s natural progesterone, they have a long list of unwanted side effects. Natural treatments such asherbal and homeopathic remedies, on the other hand, are a natural and effective alternative.
Herbs such as Vitex agnus-castus (Chasteberry) improve progesterone levels as well as promoting hormonal balance and health. A traditional Chinese herb, Cimicifuga racemosa (Black Cohosh), also has a beneficial effect on hormone functioning and may help to prevent miscarriage.
In addition, Eleutherococcus senticosis (Siberian Ginseng) promotes overall systemicfunctioning of the female reproductive system, acting as a tonic for female health and fertility. There are a number of plant based natural progesterone creams which can also be very useful in women with low progesterone levels.