Yellow diarrhea

Hello. For the past 2 years I have had yellow diarrhea usually after eating lunch. I can get it everyday or not get it for weeks. This started happening after I was in hospital for knee surgery. I have been tested for internal parasites through fecal sample twice with a negative result. I have had blood test for coeliac - also negative. The diarrhea hits me sudden with no warning and I need to go straight away. I feel fine otherwise with no bloating or flatulence. I have tried different foods for lunch and eliminated several food groups, but nothing makes a difference. It can also happen at other times of the day whether I have eaten or not. There are no other symptoms apart from tiredness. It is just watery yellow stool sometimes after I eat and nothing else. What do you think? Thanks