Straining to go open your bowels does funny things to your heart rate and blood pressure...its called the Valsalva maneouver.. essentially the heart speeds right up...sloows right down and then right back up again too in response to changes in blood pressure... "Valsalva maneuver: A maneuver in which a person tries to exhale forcibly with a closed glottis (the windpipe) so that no air exits through the mouth or nose as, for example, in strenuous coughing, straining during a bowel movement, or lifting a heavy weight. The Valsalva maneuver impedes the return of venous blood to the heart...." My guess is the extra strain this puts on on an over stretched system might be what does it... Interestingly (or not) the valsalvas maenuever can be therapuetic ( I think for some arrythmias... but may be someone who already has their medical degree can comment on that??)... Liz