Erythromycin is notorious for goofing up the digestion. All in the world that has happened to you is, antibiotics kill all the bacteria, the "good" and bad, both. You need the "good" bacteria in your digestive system, to help break down waste. So, to restore good bacteria and thus better digestion, you need acidophilus, which is the probiotics you mentioned, and you can buy it either as acidophilus milk or "Activia" yogurt, in the grocery store.