Respected user, hi I evaluated your query thoroughly.
*Frequent micturition and bladder pain is associated with 
Cystitis, 
Urethritis,
  Stone fregments irritating ureter, bladder, 
urethra .
*No it`s not like that once you have stones in 
pregnancy it will recurr.
*HCG diet will always produce stones is also not totally correct, 
*Stone occurrence depends on various dietary factors so far as concerned 
  which type of stone you are having like calcium / oxalate / phosphorus / 
  uric acid , depending on the stone type you have to avoid certain food 
  elements  in HCG diet & you will not have problem of stone pertaining to 
  that.
Hope this clears your doubts.
Thanks for using Health care magic & giving us an opportunity to serve you
  in your needed time of 
medical help.
Wishing you fine motherhood & healthy happy life ahead free of stones.
regards dear take care.