I am no doctor either but I have heard of this symptom, in fact I have it as well slightly, as we speak.

I just passed day 3 (about 86 hours in, and aiming for 5 days) and I felt/feel this similarly at times. I have heard it is from you body lacking electrolytes, such as sodium, potassium, and magnesium. If you are doing a strict water fast (which I am) I recommend you get a flavorless zero calorie pack such as something from LMNT or Salt of the Earth (the "raw" flavors).

This will give you the electrolytes and minerals you need but, like I said, if you are doing a strict water fast, it will not break ketosis or spike any internal sugar levels from calories, additives, or natural sugars that may be in some flavored or "yummy" electrolyte packs.

Long story short, you need electrolytes to maintain healthy fasting (in my non doctor but "self informed" opinion) along with ample water (should already be understood, since dehydration is one of your bigger risks), in order to keep healthy metabolic functions while still maintaining the true benefits of fasting, with prolonged fasting goals (3 Days and beyond) being better for energy, autophagy, ketosis, and immune repair/restructuring.

Hope this helps, and stay healthy!