5yrs ago I was running up stairs when I heard a pop noise from lower back&it suddenly started to hurt. Pain stayed for months&often returns after I've been sitting&try to stand up. Very sore but x-ray showed nothing. Any ideas what could have happened?

See orthopedist. X-ray is more for bone fracture than for soft tissue sprain which was likely what happened to you. MRI is better of the latter. If the pain is bothering you, you should really consult an orthopedist for an evaluation.
Resorption of disk? Herniated nucleus pulposis (or HNP) is the "slipped disk" that often has been implicated in back pain. Disks can rupture thru the "annulus fibrosus" (ring of fibers around the disk) & impinge on adjacent nerves, causing back pain. However, b4 rushing for surgery, know that these disks can also be reabsorbed, & pain go away. It helps if ur hydrated, as disks & fibers need water to keep their shape.
SIJ. 18y fem. had sudden "pop" of back followed by chronic low back pain & non-specific X-ray findings. Sacroiliac joint is likely pain generator; a disorder not able to be imaged; per medical literature. See Osteopath & have fluoroscopic guided diagnostic block of SIJ; "gold standard" test for this disorder. See Brighton Criteria to check for Hypermobility Syndrome, which predisposes to SIJ disorder. .