What you're feeling is common if you're taking large doses of PPI's. I've had the same issue as you. Chances are what you're smelling is undigested food. Since your acid is so suppressed, you may have undigested food hanging out in your stomach. When gas is released, it carries the odor of the undigested food. I'll guarantee you if you ease back on your Prevacid you'll notice that this side effect is gone. Of course, the heartburn may then be back. Six of one, half a dozen of the other.
(Note that this does not preclude CalGal's statement from being true.)
You may want to see about either cutting back on your medication (since the severity may be cyclical) or supplementation with HCL. Also, you may wish to try something called "Heartburn Free with ROH10" by a company called Enzymatic Therapy during those times when your heartburn kicks up.
Have you been tested for H. pylori? If you haven't, do so as the presence of the bacteria is suggested to contribute to bad breath.