Have you ever been prone to yeast infections? How much sugar do you eat regularly (fruit, starches, juice etc.)? It is possible that your intestinal bacteria is very out of balance and it's only the antibiotics that get it "right". Time with a good probiotic and avoiding excessive sugar consumption might set you straight (how long have you been grain-free?), but it could be more than that. I've been working on ridding myself on a yeast overgrowth for the past 6 months now. It's slow progress, but I can now eat fruit and starches with no ill effects (yeast infections or acne outbreaks). It seems that only refined sugar and wheat bother me now. There are supplements you can take to help re-balance your gut flora (clearing the slate so the probiotics can help re-build).