Yes back pack can be of stone, but then you should also have burning in urine and other symptoms of urine infection like increased frequency of urination and fever.
If it's muscle related pain then the reason could be low
vitamin D levels and this can be easily treated.
Get vitamin D and
B12 levels checked and if low get injection instead of pills.
You will be better in a week
Next Steps
consult if needed
Health Tips
take the complete course of
vitamin injections to prevent recurrence