It is likely electrolyte imbalance. Lack of salt, magnesium or potassium will do it. Or combination of all three.

Low salt will also cause low energy levels.

When you go to a low carb diet from a high carb diet, carbs tend to hold a lot more water around their molecules than protein or fat. So as the body has less carbs it also will divest of the extra water (some of the early weight loss will be fluid loss - be careful if already taking diuretics for fluid retention). As this water exits the body through urine, sweat or stool it often will take electrolytes with it, causing you a temporary deficiency. It gives some people what has been coined "the keto flu" which is not flu at all, just electrolyte imbalance.

My best guess, and it is a guess only because I am not a doctor (and therefore do not give medical advice) would be that you need more salt most likely and if not salt either magnesium or potassium. If my wife does not get adequate salt she will have leg cramps bad.

Table salt is a good source of sodium and chloride. For potassium there are supplements or a person can use some "no-salt" or "low salt" from the grocery store which is potassium chloride. Magnesium is available in multiple forms, some not very absorbable, and some that work best as a laxative. Best to start slow with any magnesium source. One way to get it is take about one third of a dose of "milk of magnesia" and put it in a glass of soda water. Milk of Magnesia is the laxative type but putting it in soda water changes its chemical structure and makes it much more absorbable and usable as a supplement rather than a laxative. Again, not medical advice, just stuff we have done and studied some.

Of course ultimately is you want to get the minerals or electrolytes you need from the food you eat rather than supplements. But early on during transition you can become temporarily deficient in one of more (perhaps all) of those electrolytes. It should self correct in a matter of a few weeks as the body adjusts to the new paradigm. That said, CB still has cramps if she does not get enough minerals. And even then has an occasional problem. But she had that occasional problem long before low carb so there is that also (and can come on at the most inopportune time late night especially during certain activities).

It has been reported that many people are deficient in potassium and magnesium.

Another bit of info. Excess potassium can cause heart palpitations and even death with enough of an overdose. So don't go eating a cup of fertilizer (potassium chloride). It can kill you in enough volume. So our government in it infinite wisdom has limited any potassium supplement to 99MG, which is a fraction of our daily needs. I guess so a bunch of us will not "off" ourselves by overdosing on potassium chloride. Yet there are no usage restrictions on "no salt" salt substitute in the grocery store. This is not medical advice but I would NOT take a whole bottle of supplement pills of potassium chloride. It might not end well. I think our bottle has 500 pills. But a single pill is not going to correct deficiencies very quickly, would be my un-medical deduction. No such restriction on "no salt". Just don't eat it by the quart jar full would be my surmise.

What I would do personally (and have done) if I got cramps in the legs is take a half teaspoon of salt, dissolve it in a glass of water, and drink it. If it appears to help, that was the problem. If it doesn't, probably not. But I am not you and you might have a medical condition where one leg would fall off immediately or you head spin or heart just simply stop and protest. So not being your doctor I would not recommend that for you. I don't give recommendations or instructions, only discuss, theorize and ponder with my high school education. Only a fool would do anything I would do without researching it themselves and making their own decision.

How's that for a cop out, LOL.