Is it normal to never find an answer about why i always feel sick?

I am always tired, and I have been dealing with chronic stomach pains and issues for the past three years. I have had numerous medical tests, seen three doctors, and apparently they cannot find anything wrong with me. I have a healthy diet - I never drink any soda or eat much of any fast foods, etc. I can sleep anywhere, anytime. I fall asleep in the car every time I am a passenger, and it's so hard for me to stay awake in class. I take daily vitamins, and even medicine for acid reflux and pain medicine for my stomach. The pain medicine is because doctors don't know why I have the pain. I'm sick of being a medical mystery. Any ideas on what else i should do?