If your hysterectomy was 20 years ago it is exceptionally unlikely that it has anything to do with your current problems.

the first and most common cause is poor pelvic muscles.
this can be helped by doing kegel exercises.

the other possible and probable cause is a prolapsed bladder which can be helped with kegel's as also with a bladder sling operation.

there are of course many other possibilities but with out you being seen by a qualified medical professional we could suppose forever.