We got our first bunch WAY too early too. They should be about 40-50 lbs when you get them. Trust me, you'll have lots less trouble with bigger ones.
However, that doesn't solve the problem at hand. If they aren't sneezing, coughing and showing any other forms of illness, I'd say it was the food. They don't take to switching food well, you should mix old with new until they get used to it.
You've also got to be very careful with dehydration right now. Some feed stores sell Swine Nutri-Drench and a few treatments of that helps some. We also fed ours lots of oats for a while. Now, it could be Cocci too. Similar to what chickens get, but caused by a different Protoza. If I were you, I'd get them back on their old feed, add in a bunch of Oats, try the Drench and see how they are tomorrow. If you are like us, you can't afford a vet for a $40 pig. Make sure they are drinking a lot of fluids. Dehydration can take them very fast. Also, don't feed them any squash or anything for a while.
Good luck.