Interview went well- why haven't they sent me the assignment?

Hi had an interview today, which seemed to go really well. Lasted over an hour, they were happy with my work examples, they seemed to like my answers. Before I left, they said they would like me to do some homework for them to see what my writing style is like. They said to send it back to them asap. I didn't ask whether this would be via email, as I presumed it would be. The interview was at 11am this morning, it is now 7pm. I sent them an email a few hours ago, thanking them for their time and checking this would be through email and that I didn't forget to take a paper home with me, hoping this would get some response. I have received nothing in response. I know I am coming across as very eager,as I really want this job. Am I being too stressy about this, or is there likely an issue that is preventing them from going ahead with me?