I really wish Mark would do a blog post about this since he just recruited so many newbies.
If you reduce your carbohydrate intake (and face it, everybody new to this has reduced them if they gave up wheat and grains), then your body is going to work completely differently in regards to sodium and hydration. Your kidnesy will dump water and with it, sodium and potassium. This will make you feel horrible and crampy. This water comes from all the glycogen your body was holding onto with your former high carb diet. Your kidneys were holding on to potassium. Now it all comes out because you don't need it anymore. You get a little out of balance at first until you adjust.
You need more sodium. You need to make sodium a regular part of your diet. It goes against everything common wisdom tells you. It goes against your doctor. But you need it. Your body works differently now. Sodium will help you feel better and it also helps maintain your muscle mass.
You may wish to take some potassium supplements and some magnesium (citrate, malate, *-ate) supplements, too. Do this for a while to help your cramps and make it easier to sleep. Do talk to your doctor first if you take blood pressure medication. You don't want to take potassium if you do.
Salt your food, learn to make bone broth, lick a packet of salt off your hand and chase it with a glass of water. You'll feel much better.
You can also feel better consuming more carbohydrate foods, but at this stage, your diet is so much less in carbs just from removing grains that you probably can't make up for all the current discomfort by adding fruit and potatoes. Even with more fruit and potatoes, your body is going to work differently than it did in the past.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 170 x 3. Current Deadlift: 220 x 3