If I remember right your supposed to get in every 12 weeks. What dates did they give you at your last shot? If you think it is too late then call and ask them to look and give you the correct dates. Then ask them to give you appt in that time frame so that you get it on time.
If you don't get it in time. Don't have unprotected sex before you get it. And ask them when you go in to get it if you need to use backup for the first week to be protected again.