Can there be something more besides H-Pylori and IBS?

I've been suffering from abdominal pain, heartburn, bloating and constipation for about two years. Usually these symptoms would come and go, but in the last half year they have become more and more severe, until I went to a doctor. I've been found to have a H-Pylori infection, which after two weeks of antibiotics, turned out negative in a stool test.

The abdominal pain has disappeared. I am now avoiding things that recently only have been giving me heartburn, like coffee. However the bloating and constipation are still giving me hell. I was diagnosed with IBS, and tried all sorts of remedies, from drugs to laxative teas and charcoal tablets, but nothing works. After three months of severe abdominal distension and constipation, I am getting more and more irritable and quite depressed. I am on a very healthy diet, I exercise, I don't feel stressed, I am young in my late 20's, and have no other health problems that I know of. What could be the cause of it?

Thanks,
Maria