I feel the doctors aren't diagnosing me properly. I'm 18 and male

In July I was having heartburn and chest pains everyday and eventually went to the a and e. They took 3 blood tests, an ecg, ekg and chest x ray and said all my results were perfect. They said I was probably stressed. 
 

Since Friday I've been having horrible stomach pain and cramps. My stools are normal. No blood or anything but no matter what I take I still have stomach pain. I feel I should ask for a colonoscopy this time around. Nobody in my family has ever had cancer before, same with extended family and I'm healthy and fit and eat clean