Got epidural about 5 months ago during labor. Ever since I've had bad back pain. Lower spine feels like there is little rocks in it or knots. Serious?

Not related. Epidural itself is unlikely cause. There are many reasons for back pain: strain, herniated disk, spondylitis , kidney problems, sciatica, arthritis among many others. See physician for evaluation.
Common misconception. That epidurals somehow cause lasting backache. The backache is from the pregnancy itself. During this period large amounts of Progesterone lead to ligament laxity everywhere, in the spine, pelvis and hips. After delivery women who don't strengthen their core or don't address the chronic psoas muscle spasm will frequently complain of chronic back ache but wrongfully attribute this pain to epidural.
Unlikely if no other. Back pain can be caused by many of the physiologic changes related to ligament laxity, position during delivery, descent of baby down birth canal, as well as changes related to presence of catheter. Most pains usually improve within a few months. Some may last 6-12 months. If no additional neurological symptoms, i would recommend therapy and improving core muscle strength as first steps.
See below. Low back pain is a well recognized "side effect" of pregnancy. It has never been shown to be related to epidural placement. I suggest you see your pcp or ob/gyn.
SIJ. Earlies twenties female with back pain radiating to buttock 9 months after delivery and epidural should be evaluated for disorder of sacroiliac joint (sij). I have seen same pain pattern in several women with epidurals preceding severe chronic low back and pelvic pain of sij disorder. My speculation: absent proprioceptive-driven guarding, the pelvic ligaments were injured during birthing process.