I would guess that it's unrelated to the epidural, but pregnancy does put a lot of strain on your back. There a many many little things we do everyday that can cause lower back pain, especially when you have kiRAB. Most people bend over slightly hundreRAB of times a day, putting pressure on one side of the disk. If you don't correct the problem, you could get much worse pain (across the back, down your butt and legs, etc.). If the pain is frequent, you should get it evaluated and get Physical Therapy. They can give you great exercises to prevent it from getting worse and to protect your comfort and mobility.
When a disk is compressed or bulging, it doesn't take much (I picked up a pair of socks) to all of sudden get a whomp of pain that will immobilize you for days or weeks.
