Not sure that I agree that you should get an MRI. The majority of asymptomatic adults have abnormalities such as "disc bulges" on their MRI. Therefore, it is difficult attribute back pain to any particular abnormality on an MRI. Rarely, an MRI may demonstrate a stress fracture, or spondylolysis, but this should show up on the X-ray you had. An MRI is really only useful if you have pain referred to your legs. Even then, the abnormality, such as a disc prolapse, must correlate with your symptoms. For example, if the MRI shows that you have a disc prolapse compressing the right S1 nerve root, but the pain is in your left shin, then it ain't the cause. Unfortunately lumbar back pain is extremely common, and 80-90% of us experience an acute attack at some point in our lives. The important thing is rule rule out sinister causes, e.g. fracture, infection, cancer - that is what we all worry about, but it almost never is. Pain associated with these problems tend to be constant, unremitting, worse at night, perhaps associated with fever and weight loss. These are the so called red flags.
on a final note, I had acute back pain last year, and it is bloody awful. Good news is the majority of back pain settles spontaneously. Hope it gets better soon.