I feel cramps backach fatigue after my periods and same condition almost from 20 days and now start bleeding at 12 while my mensuration date is 20.

Rest and Observe: Few cycles a year a little different than most of your cycles. Every cycle is a new hormonal cycle. Sometimes, you will experience emote cramps than usual and your bleeding pattern may change. Take some Ibuprofen and watch. If the cycle persists this way for 2 cycles then see a doctor.If you have a new sexual partner, then cramps can be from infection. Do not wait and see a doctor ASAP.