When a BM happens just after eating, it indicate that you are eating something that is a trigger food that causes you to have a BM. It is usually IBS -- and everyone that has it has 1 or more trigger foods.
My wife could not even get finish a cup of coffee before --diarrhea!!
You have to find the food/drink and elimiate them from your diet.
Is it very time you eat? or just in the morning? Is it only after a large meal or after a bite of only one thing?
Eating triggers peristalsis....whatever stool is in the lower sigmoid will be then "downloaded" into the rectum.
Foods don't go through the digestive tract immediately, so it's not an offender if you've just eaten it. You might feel nauseated after immediately eating some foods which will trigger the peristalsis....but that food still has to go through digestion.
Coffee is a stimulant....again, it stimulates peristalsis. Many use it in the morning to encourage their bms.
It's all a matter of perspective!
As was my experience, I had rapid transit of food along the digestive tract as evidenced by a 20 minute Small Bowel results. I had had long standing GI issues and until I saw the last GI doctor (#8 for me) I assumed it was IBS related. Well after a host of tests, it was found that my small intestines had suffered an insult to the villi due to longtime Antiinflammatory use. It has taken about 4 years of no lactose, no fatty or fried foods and an antiacid med (Acihphex) to make the situation better. I too could eat and the food remain unchanged throughout the digestive tract, such as whole foods appearing unchanged or medications that passed through the tract intact.
I would see a GI doctor who specializes in IBS or rapid transit. There are ways to slow the food down. I know I feel much better now that I have the diarrhea and rapid transit more manageable. I still have periodic episodes, but not the constant pain, cramping and diarrhea I did at its worst. I also take prophylactic Immodium each night ranging from a half a tab to 6 tabs depending on the previous day's bowel activity. I do hope you find a great GI doctor, as the constant pain and irritation of the digestive tract can lead to other problems, i.e. weight loss, loss of nutrients, etc. This was my experience. I do hope you get some answers and some relief.