Nightly leg-muscle cramps are commonly used by a lack of magnesium.
Flooding yourself with water might have an adverse effect because you also flood your electrolytes away, so your low magnesium level gets even lower. Your dietary change may also have been a major cause, if your new diet is too low in magnesium.
Adding calcium is a bad idea. Not that calcium itself is bad, but calcium and magnesium are chemically similar (both forming ions that carry a 2+ electric charge). Therefore, increased calcium levels in your intestines result in a lower uptake of magnesium.
The best you can do is eat food that’s high in magnesium. Bananas are the best-known example.
For a quick fix (changing your diet does work, but it takes time), you could temporarily use magnesium supplements. I’m not familiar with those; I do know that some supplements are more easily absorbed by the body than others, so maybe other readers can advise on which kind of magnesium supplement is the best to use.
Hoping this helps,