hi good afternoon. thnxs for your Question. i think pills that u took may be cause of youe irregular periods. because pill contain hormones in low dose so if u take it in proper timing it will help otherwise give side effect on your body and on your periods.  but dont worry  now u  just wait for your next menes comes if its normal than no worry. and regarding your backache  i wana know if you have any whitish discharge from vagina in betweeen your periods?  if yes than let me know in detail abt it . i hope this will help u.  stomach bloting is also a side effect of pills. but it will reslove.