The symptoms you describe almost perfectly match those my husband has. He started having these symptoms in his early 20s. He's 30 now, 6'1", 165lbs (BMI of 21.

. He's never had an endoscopy, and has never received an entirely satisfactory diagnosis. The closest thing he's found that seems to be roughly applicable is Irritable Bowel Syndrome (IBS). A lot of the dietary guidelines they suggest seem to apply, although most sources that describe IBS fail to mention nausea, which is a symptom you and he both experience.
For him, the big no-no's are 1) STRESS, 2) LACK OF SLEEP, followed by 3) HIGH FAT (e.g. alfredo sauces, fast food), 4) CAFFEINE, and--to a far lesser extent--5) alcohol. He also suspects that high fructose corn syrup might be a contributing factor. These are particularly problematic when they are combined. For example, going to a stressful meeting and drinking caffeine (and, heaven forbid, eating an eggs/bacon/sausage breakfast) after a short night--especially one that involved a lot of alcohol--would be awful. He HAS said that at night,
one beer can actually help to relax his stomach, and that if stress can be avoided--ESPECIALLY if he can get 9-10hrs of sleep a night--his tolerance to the other factors goes way up.
You said you don't overeat; that's good for him too--small meals are more tolerable than big meals. The opening of a Trader Joe's (grocery store) in our area has been a godsend in our lives, as it has made it easier and more affordable to eat well. If stress/lack of sleep can't be avoided, he either chooses not to eat at all or tries very bland carbohydrates like plain rice, bread, crackers, etc. But even so, the problem sometimes defies all explanation, occurring even when he's been "good." Not only is it painful, but it becomes extremely frustrating and discouraging because of its unpredictability and the impossibility of controlling it completely.
The only test I can think to suggest is perhaps a blood sugar analysis. I've heard that high blood sugar (like you might get after a meal if your blood sugar is not well-regulated) can lead to the irritability/fatigue symptoms you describe (but not the intestinal symptoms).
You have our empathy, and we wish you the best of luck in finding a way to decrease your symptoms. If you discover any ways to improve your condition, please let us know. You're not alone!