Night-time diarrhea

Question
We have a 4 month old retriever who has been housetrained well. However,
just these past 3 weeks suddenly she is suffering from diarrhea during night!
Whole day she will eliminate hord stools, but recently she gets up, atleast
twice, during night, in pain and with an urgency to go and passes loose stool.
There has been no change in her diet, she is fed Science Diet for Large breed,
3 times a day. ONLY change is that we have started her on puppy
socialization classes where she and we learn about training. Also, she gets a
chance to play with other puppies during these classes. Could the high
activity of the class (?stress) be causing this diarrhea? Why it happens only
during the night? Her stool exam results have come out normal. We have
given her boiled rice/boiled hamburger meat to ease the diarrhea but the
pattern has not changed. Will be most appreciative of your feed back. Thanks.

Answer
I would mainly urge you to push the vet for more tests.  Are you using treats in the training class?  A number of treats in the evening could lead to problems if she doesn't tolerate them well.  Most retrievers are well enough food motivated that a little of their regular kibble works fine as a treat.