Gastrointestinal problems?

For years I've had IBS, usually bouts of a few weeks and then subsides, so the doc assumes anything going on down there is IBS and suggests over the counter meds. However, over the last few weeks I've had a real problems with my guts. Huge amounts of gas. Noisy guts. Foul smelling, loose stools (not diarrhea as such, just soft/loose). Nausea (queasy rather than needing to throw up). Dizzyness. Dull pain in the abdomen.

As the doc won't do anything and I'm so uncomfortable, I'm thinking of some antibiotics from other sources just to try and kill anything off that might be lurking in my guts. Now, I don't believe in antibiotics unless they are absolutely necessary - it's been years since I last had any. But I'm so fed up that I'm willing to give it a go.

Any suggestions on:

(a) what it could be (it seems more than just IBS)

(b) whether antibiotics could help

(c) which antivbiotic and what dosage (I'll double check this myself)

Thanks