What Causes Diarrhea Every Night At The Same Time?

I have had diarrhea every night at the same time of day for about 10 days now, usually 1 or 2 visits to the bathroom. I have had some diarrhea problems in the past but all checkups and a few colonoscopies turned up normal. This happens very suddenly and I just make it to the bathroom. I am a 65 year old female.