What is the most likely cause of my recurring lower back pains? Arthritis? Disc?

Muscle, joint or ? There can be many reasons for lower back pain. Muscle can be a very common cause of back pain and that is where exercise can play an effective role. There are arthritis causes that can be inflammatory or degenerative changes of the facet joints of the back. Disc disease can compress nerves and also cause pain but despite its prominance in discussions it is not a common cause of back pain.
Muscle weakness. While there are many causes of back pain, i find that recurrent pain (as opposed to constant pain) is due to muscle fatigue and responds well to physical therapy.