I can almost guarntee your lower back issues are mobility-stablity related and are made worse by your training. Simple approaches to get rid:

Make your hips more mobile, stretch your hammys, hip flexors (this is a common cuplrit), quads, calfs, the lot. Also do thoracic spine stretches, search for some online, i think thoracic extensions are best. Do not do any sit ups or crunches, these cause flexion of your lumbar spine which will make your lower back pain even worse among other things. Finally record your lifting form, particulary when picking thing u p from the floor, whether thats deadlifting or just picking up db's for shoulder presses. A flexed lumbar spine when doing this will really stress your back.

Following the above will definately help, if not cure it, however it will take lots of time and work and if it is more of a clinical injury (i.e. slipped vertebrae) it will need professional help. I suggest seeing a physio, they will refer you to a chiropractor if needed.