Hi totallyterrified,
antibiotics are antibiotics - doesn't matter if they're penicillin or any other kind of antibiotic. They kill all good bacteria because they don't differentiate between good and bad, which is why many people develop yeast infections. The best way to combat it is to have PLENTY of yoghurt (but plain yoghurt, not one full of sugar, chemicals and flavours, or better still, go to your local health food shop and get probiotics, keep them in the refrigerator (because they are freeze-dried good bacteria) and have one every morning on an empty stomach. Most doctors in mainland Europe now prescribe probiotics when they prescribe antibiotics.
And for the time being, stay well away from sugar, all fruits, fizzy drinks, mushrooms, tomatoes, and anything you know would grow fungus on it if left out in room temperature for a few days - because if it will do it at room temperature, it will surely do it inside your intestines!
You might also want to google candida albicans, which is a systemic yeast infection, because if you respond that quickly to penicillin it's possible you have been harbouring a non-symptom yeast infection for a while which can result in fatigue, bloating, and lots of other symptoms.
Sorry, not trying to be a downer here, but as a trained naturopath I regularly deal with people who have systemic yeast infections. The upside is, it's not fatal, and it IS treatable, although don't expect to find too many GP's who will recognise it.
