Symptoms after cat has iodine treatment?

My 15 year old cat had iodine treatment last week on Monday June 16th and I brought him home Thursday afternoon. He was acting fine on Thursday and Friday, eating normally and drinking water, but on Saturday and since then he has been throwing up just like he had been before treatment. He shows some interest in his food, but won't eat much. Plus, he is always lethargic. I heard that lethargy and decreased appetite can be side effects of the treatment. Has anyone who has a cat that has gone through this experienced this as well? How long did it take for your cat to get over these symptoms?