Potassium is an incredibly important nutrient. It aides in the communication between the nerves and mucles, regulates blood pressure and also helps the body to flush fluids.
Potassium loss can play a role in things like edema, water retention, hypertension and other bad things.
Potassium is found in many foods, including artichokes, beans (lima, kidney and a few others), greens, nuts, parsnips, tomatoes, squash, yams.. and many more, but not all of them are acceptable on every low-carb diet.
As for carb withdrawl symptoms - usually for the first few days of a new low-carb diet, many people experience something like the "Atkins Flu". As the sugars leave your body and your body switches to burning fat for energy, you can feel tired, crampy, cranky, just generally blah.
That usually ends within 3-5 days, though it can take several weeks for some people.
Anyway, welcome to the board, this is a very cool place to hang out, nice to meet you!
Morgan