I was doing standing bar twist(bar on back shoulders) now my lower abdominals feel pain when bending at waist, what?

Possible: abdominal muscle sprain. rest, ice and analgesics

A good workout?: Sounds like you were successful in working out your lower abdominal muscles and obliques. That pain should subside slowly over a few days.. If you pulled a muscle the pain is usually more severe and localized to one side or the other. If that's the case then see your doctor.