I had back labour without an epidural. I'm not sure if there's a correlation but I would get back aches before my period every month so maybe if you have that you're more prone to back labour!
Anyway, it feels like your back muscles tightening up as much as they can and it wraps all the way around your abdomen slowly.
For me, I could handle it. It was terrible pain but the more I relaxed, the better I got through it. Everyone handles the pains differently.
As far as your pain, it could also be how baby is sitting or could be pinching a nerve or something.
I just gave birth to my daughter a week ago and I had back labor. And oh my god it was the worst pain I've ever felt. Thank god for epidurals. Its an intense pain in your lower back right above your butt. It's like a tightening feeling and will work it's way to your abdominal area. I didn't realize what it was untill it got intense enough tht I couldn't talk. Good luck hun!