I've often noticed that I sweat a lot during and/or just after bowel
movements - mostly on my forehead and torso, to the point that I get
droplets running down my forehead and my clothes become damp.  I'd
like to know whether this is a common effect, what causes it
(metabolic changes, exertion, etc), and, if it's uncommon, whether
it's associated with any particular disorders.

I presume the perspiration is, as usual, a symptom of an increase in
body temperature, but maybe that's not the case.  I have not actually
tried to measure my temperature (I never have much luck with
thermometers, I always end up with ridiculously low readings).

I'm a man in my late twenties.  My diet is not very healthy, but I
don't suffer from digestive problems, diarrhoea, constipation, etc. 
I'm around 10% overweight - I'm aware that this can increase
perspiration in general (not sure where I read that though).

I don't exert myself much during bowel movements - I know some people
do, but I mostly just sit there and let it happen.

The Wikipedia article on "Defecation" lists some side effects of
defecation, including raised blood pressure, cessarion of respiration,
and lower heart rate.  At least some of these would seem to imply a
reduced metabolism, though, not a raised one.

The Wikipedia article on "Thermoregulation" lists some causes of
variation in body temperature - diurnal/circadian variations and notes
that meals, alcohol and exercise affect overall body temperature, but
does not mention defecation.

Another possible cause that sprang to mind was the restricted blood
flow due to sitting on the toilet seat.  Not sure whether this is the
cause - again I'd expect a lowering of metabolic rate.  I'm clearly no
expert though!

Thanks for any answers to these questions.  To reiterate from the first paragraph:

* Is it common?
* What causes it, biologically?
* Does it indicate a particular disease or other disorder?