I Have A Question About Consumption Of Olive Oil And

I have a question about consumption of olive oil and severe diarrhea. I am 63 years old. In the last 8 years, most of the time when I order a salad with italian dressing at a restaurant, I end up with diarrhea and intestinal cramping before I leave the restaurant. I have had to quit ordering those salads. I cook with small amounts of olive oil and have not had a problem. I sometimes use small amounts of olive oil with herbs as a dipping sauce for bread and have not had problems. I'm trying to figure out if my problem is the oil or something else that may be on a restaurant prepared salad. Is it likely the olive oil or something else that is causing this problem for me? Thanks.