I have heard of this often! Here are some things that have anecdotally been the problem for some people:
Electrolyte depletion - get more salt, potassium, magnesium, and chloride. The body dumps a lot of lytes when transitioning to very low-carb. Mine was temporary, salt fixed it, then I wasn't eliminating as much after a few months and ended up with high BP from the excess salt. So now I just get my salt from bacon and sauerkraut and pop a little magnesium now and then. You may also be low in iodine, vitamin D, or any number of pet nutrients this place has grown attached to. I STRONGLY recommend eating a good half-pound of liver once or twice a week for all the B vitamins and others, and some seaweed for the iodine again once or twice a week. Bone broth is also easy to make and did help me, and I still make and consume it somewhat regularly.
Punch your foods into fitday or a better food-tracking system and closely examine your mIcro nutrients very well, and find foods that wil help you - over the course of the week - to fill everything in. Between liver, greens, nuts and eggs, I think you'll find most of it covered. Oh, bacon is not only a great source of zinc, copper and choline - it's also one of the best sources of thiamin in the world. Check out beef heart, too. Tastes like beef, you can get it cut into steaks, and it's easy to store and cook at the drop of a hat.
Carb cycling - Some people find it easier to maintain a good mood when they add carbs back, and smaller number of them may gain or lose more body fat when they do this. To offset those complications, many enjoy periods of normal-carb eating with windows of very low-carb mixed in. Say four or five days of one, two or three of the other. This worked amazingly well for me, while some find this unsustainable due to difficulty switching between fat and carb metabolism. Intense, infrequent exercise should fix that.
And it may not be food-related. Look at your whole life. Does something need to be resolved so you can clear your mind? Address it!