Thank you for the information.
Sometimes when the bladder descends down from its normal position onto the vaginal wall(called as Cystocele),it fails to empty completely resulting in this particular symptom.This may also result in Stress Urinary Incontinence(dribbling of urine during laughing,sneezing etc) as the anatomy of the Urethra too changes.
I feel that this may be the reason in your case.
This can be either due to weakening of the ligaments supporting the Anterior vaginal wall or the pelvic tissue itself,but not because of a problem in the bladder.
The treatment depends on the severity and accompanying symptoms(Kegels as well as Physiotherapy or Surgical in severe cases).
A lack of Estrogen may too cause weakening of the pelvic tissue resulting in this symptom and use of estrogen may help(Mostly seen in menopausal group which may not be the reason in your case).
Here either a Urologist or an Ob/gyn can offer treatment.So, you could just see your Ob/gyn to see if you have any apparent Cystocele.You may not feel it if it is very early stage and may be evident only on straining.
I hope this helps you.My best wishes to you.