It sounds like you may be suffering from a bout of low back pain (LBP) called nonspecific LBP. Most likely it's caused by some soft tissue problem but the exact mechanism of pain is unknown.
In cases like this the physical therapist relies on a classification method to decide on treatment. Treatment is determined based on your signs and symptoms. For example if you are 40 years old or younger with full hamstring motion and excess joint motion, then stabilization exercises may be best.
For patients with very recent symptoms and loss of joint motion, manipulation or manual therapy is advised. And for patients whose symptoms are better with certain motions and worse with others, specific exercises may be needed.
The therapist conducts an exam looking at all the various factors known to impact treatment. Specific treatment of this type often speeds up recovery. Once your symptoms start to improve you may be progressed to the next level of exercise.
A program of low-stress aerobic exercise on a bike or treadmill is common. At that point exercises to correct postural faults, muscle weakness, or inflexibility may help prevent future episodes of LBP.