Have you tried Ispaghula husk? It is more natural and works better than any medicines.
I'm sorry to hear how insensitive your doctor was. Some doctors just don't understand their patients, that they are in pain, and that also pain should be treated.
How do we know if it is something else? We just have to trust our own judgement I suppose, and if we are not certain, go to the doctor. Even if we have IBS it doesn't mean we mustn't go to the doctor! However, if you've been scoped 2 years ago and everything was normal, I doubt you need similar examinations now. Maybe you still should consider finding another doctor.
When doctors fail in what they're supposed to do they think it is between OUR ears http://www.healthboards.com/ubb/bang.gif