<< Also, I just noticed that if I twist my body (like I do when warming up in hockey), I feel pain. >>
You should never do that twisting motion as a warm-up or for any other reason. It puts a lot of undo stress on the Lumbar spine(low back). The lumbar vertebrae are designed to only bend for flexion and extension. Any twisting motion here occurs by shearing forces(not good).
Sounds like muscle strain only, which is good. Use moist heat on the area. Also, lay on your back with knees bent, pull knees(both) to chest, hold for 5 secs, do 10 at a time, several times a day.