I have lower back pain, in the lumbar area. it is typically more so after i have been sitting, but is relieved when standing or laying down. its pinpoint but radiates across the lower back, with associated lower back spasms. I will have pain for a few weeks at a level of 8/10 then I may not have pain for months, but it is on going and has been for 5 years now. it is also completely random as to occurrence. i am not over weight and i am physically active.
Unfortunately, medicine is still lagging behind in knowing exactly what the pain generators are in the back. It is known that when a nerve is pressed on, by whatever (herniated disc material, enlarged facet joints, bone spurs, etc), that the pain radiates down the leg. This is referred to as nerve pain. You do not describe this type of pain. Sometimes, spinal stenosis (narrowing of the lower spinal canal) can also cause radiating pain, but this is usually around the outer hips, into the front of the thighs.
As to pain located in the back it could be just within the muscles. Or, with the increased use of MRI studies, degenerative disc disorders have been found in more people. But, it is still unknown how much the MRI findings correlates with a patient's symptoms. Arthritic changes between the vertebral bodies or of the posterior facet joints can cause back discomfort.
If it bothers you significantly, impacting on your daily activities, see a physician. Since you have tried the conservative route (OTC meds, keeping the core muscle strong, PT, etc) you may want to see a specialist, such as an orthopedic spine surgeon or a neurosurgeon.