Please help 15m old having bad Diarrhea.

My son has been on formula sense he was 3 weeks old (Enfamil Gentle ease) finally hes 15m old. So when he was 12m we finally decided its milk time. We switched him to milk and he had watery yellow diarrhea 5 times a day for about 3 or 4 days. We put him back on formula because we thought to wait longer.. k now hes 15m and doc told us to try Soy milk so we are. 2 days in and today my son had watery yellow diarrhea pouring out of his diaper it was down his legs to his feet and he's been SUPER gasy. I'm going to wait a few more days to see if it gets better and if not making an appointment again. has this happened to anyone elses baby?