I've never had stomach problems on stimulants, but that doesn't mean much. Everyone's different.

Standard Ritalin will give about the duration of effectiveness you describe. 4-6 hrs max. When I was taking IR versions of those, I would take them 4x/day.

If you and your docs determine you do have ADHD, there are a lot of different stimulant options to try. Ritalin and Adderall are the gold standards. They're different medications; Ritalin is methylphenidate and Adderall is a mix of amphetamine salts. That being said, they behave very similarly and usually one or the other is better for an individual's symptoms. Then there are multiple extended release concotions of each, simply because Ritalin lasts for about four hours and Adderall about six. Everyone wants to find the purple squirrel; the one formulation that's effective all day and doesn't keep you up all night. Newest on the scene is Vyvanse, which is actually a different medication - it's a pro-form of an amphetamine that doesn't get metabolized to the actual drug until it's in your system and your body cleaves the molecule. The upside to this is it has a very long effective window - for most people it lasts 12-14 hours. It can also be a bit less "jarring" on the upswing and letdown when compared to the immediate release meds.

Regarding your mental reaction to taking the Ritalin, it sounds like you had a productive time aside from the stomach upset. That productivity is exactly what you're looking for with a stimulant treatment. The comparison to Red Bull is decent, but it shouldn't be as much of a "jittery" feeling that energy drinks can cause.

Good luck, and keep letting us know how it goes!