When a person (especially a child) has a stomach bug, it is very common for them to have a relapse of vomiting. You thought they were all better and then they throw up again. This is why it is very important to keep kids home from school for at least 48 hours after the last episode of vomiting because the vomiting frequently starts up again. It is best to keep them home until they have eaten normally for a full day and have not thrown up. This happened to my son, Michael (age 3) in February 2014. He he vomited 10 times on Friday. Then he vomited 1 more time on Saturday (22 hours later). Then on Tuesday night when I thought he was all better, he threw up a huge amount again (3.5 days later)! After a stomach bug, it is common for a child to barely eat any food for a few days. When they finally start to eat a little bit again, their tummy may reject it and throw up one more time. Sometimes dairy products are to blame for the relapse because they can be difficult to digest after a stomach bug. Occasionally, a person will have a relapse of vomiting about a week later! This comes as a horrible surprise to the parents who thought the child had been well for the past 6 days. Usually, it is a very mild relapse with only 1 or 2 vomiting episodes or diarrhea. Very rarely, the relapse occurs 2 weeks later with vomiting or diarrhea. If the person seems very sick again, please consult a physician.
Unfortunately, people don't develop good long-term immunity to norovirus, so it is easy to catch the same bug again a few months later.
--Annie Pryor, Ph.D.