There's nothing wrong with homemade yogurt, and it's pretty hard to screw up. Not all cooks are equally careful with food preparation, so sometimes it gets moldy or contaminated additives/fruit/whatever are used. I wasn't suggesting that homemade yogurt is a problem, just pointing out the (very unusual ways) it might possibly cause your problem. It's more likely that something else is doing it, though, unless it's lactose intolerance.