How Is it right to get arrested for poss.of conrolled subsance,when i was stopped for speeding?

It sounds like you were stopped for speeding, they ran your record, and must have asked for consent to search your vehicle and found a controlled substance without a prescription to that controlled substance. Once you are stopped for reasonable suspicion, then further investigation can be warranted. You may wish to seek the advice of the attorney you previously hired with which you are on probation for.  I have concerns that this new arrest may be used to revoke your current probation.  Get to an attorney as soon as you can to discuss this matter.