Hi
Welcome to the MedHelp forum!
As Caregiver said, it could really be electrolyte problem. When you start band, keep an oral rehydration with you which you can mix in water and drink 2-3 times everyday.
If this does not help you, then this can be due to a pinched nerve in low back, less likely at your age, but not impossible. Take care!
I have had a similar problems just in one leg, only it was a tendon from the back that had shortened due to an operation. I have had to have physiotherapy and chiropractic treatment to correct this. But probably for you it is just cramps from lack of the correct nutrients in your system as caregiver 222 has answered.
The most likely causes revolve around electrolyte depletion, calcium depletion, magnesium depletion or potassium depletion. All necessary for proper muscle contraction. Try drinking some Gatorade or similar electrolyte. If you try a calcium supplement (or milk) take it with D3. Eat a banana (potassium). Inexpensive magnesium glycinate supplements in small doses have no contraindications. A co-factor is insufficient circulation. Not likely at a young age. Try 1000 mg of Omega-3 fish oil supplements once a day.