I wouldnt' be overly concerned but I would watch. I was stuck at about 2cm for weeks, so I know it can take awhile, or it can happen fast. My hospital typically does not admit until 5cm unless there is a complication and I do believe that is fairly common practice. I also agree with ladyhenry that what you could be experiencing is back labor. However, if it's still not too extreme, things are probably fine. However, any concerns (and yep, I would consider possibly missing out on that epidural a concern), head into L&D again.
Sounds like any minute now your going to be holding that baby! Huge congrats to you.
From my understanding and reading a lot of post and books, back pain can be labor. My first pregnancy I had extremely painful cramps, my second was more of a pressure type feeling but still painful but ended up having to get a c-section because of her heart rate dropping during contractions. A lot of women said they had their contractions in their back. Hope this helps a little. Good luck and congrats!!