From the symptoms you are describing above, a couple of things can be thought of as a cause
1. ?Irritable Bowel Syndrome ( IBS )
2. ?Lactose Intolerance
3. Any other disorder of malabsorption
Regarding the urge to defecate when you have stress, don’t worry about that; that’s a natural response of the body. Processes are controlled by nervous system and one affects the other and vice versa.
So that is not an abnormality. Once stress is reduced, this problem will resolve.
Next Steps
A thorough history and review of lab reports is required Since colonoscopy is normal we don’t have to worry about a lot of causes which lead to such problems so that is indeed a relief.
Health Tips
1. Maintain a diet with low FODMAP ( Please Google )
2. Avoid all dairy products for one week and see if there is any change
3. Avoid caffeine and nicotine intake for one week
4. Maintain a proper sleep hygiene and focus on reduction of stress
5. Consult for a detailed and thorough work up