This is a serious thing. The discs shrink during the day due to gravity but grow after sleeping all night. So being horizontal is the best thing for it. So you could swim since in water you are almost weightless.
Get plenty of sleep now and the rest of your life. You can exercise, eat, sunbathe and drink water too much but you cannot sleep too much. Why not? Because it is not in your control. If you do not need sleep you will not be able to do it (not talking about the stupid stage 1 sleep that is not sleep). Try sleeping for 10 hours after getting a full night's sleep of 9 hours. You will not be able to do it. Why is this so hard to understand? You can't do it if you do not need it. Try to vomit on an empty stomach. You can gag but you cannot vomit.
You could also use wet heat while lying down to help it heal. Stretching is also good for the back like yoga. This is for later on and the rest of your life to prevent this from happening in the future. While you are lying down the discs expand with water and nutrients entering them. So make sure to get enough nutrients like omega-3s.
Also you can see a special chiropractor that knows how to measure what is wrong and by how much. See this site
for that. It tells about the governor of CA, Barry Bonds and Lance Armstrong going to a chiropractor regularly.