It seems that you may be having the symptoms of stress incontinence of urine. This occurs when the pressure in
the bladder exceeds that on the urethral opening. There may be small leaks during activities which increase
abdominal pressure, such as laughter, sneezing, coughing, etc.
It is possible that the neck of the bladder has descended (dropped downwards). I am pasting an extract from an
article on this topic. This extract is published by Dorothy N. Kammerer-Doak in http://www.femalepatient.com/html/arc/sel/oct02/article05.asp
"... Two possible causes of genuine stress urinary incontinence (GSUI) have been postulated relative to
how the bladder neck functions to maintain continence with increased intra-abdominal pressure.......When the
urethra has descended or is hypermobile, increases in intra-abdominal pressure are transmitted to the bladder
but not to the urethra. Bladder pressure may then exceed urethral pressure, resulting in GSUI....."
In the absence of any indicators of infection, stress incontinence should be considered to be a result of
descent of the bladder neck.
Some more suggested reading -