The ER probably would just tell you to go there, and that may take a while.. I know ER visits have always been time consuming and frustrating for me.
I know it is a holiday weekend, but try getting in touch with the doctor who prescribed it to you. Most doctors offices have nurse lines you can call and get help from even when their offices are closed. Your local hospital/ER may even have a hotline you can call. It is worth a shot anyway.