Brief Answer:
Can be a back muscle spasm or posture related.

Detailed Answer:
Hello,
Thanks for posting your query.
Lower back pain is quite COMMON among middle aged individuals. Back muscle strain is one of the MOST common causes of acute to severe back pain. Lifting a heavy object, twisting, wrong posture or a sudden movement can cause muscles or ligaments stretch or develop microscopic tears.

Some other underlying causes like Sciatica, Spondylitis, Lumbar herniated disc, Degenerative disc disease, Isthmic spondylolisthesis etc should be ruled out through clinical examination and radiological investigation (like X- ray, CT scan, Myelogram, MRI etc).
TREATMENT: Some general guidelines can help you to reduce back pain. These are as follows:

1) Take adequate bed REST with maintaining posture. Individuals who do not regularly exercise to build strength and flexibility are more likely to experience recurrent or prolonged back pain.

2) Alternate Hot water bottle or Ice pack can help to relieve most types of back pain by reducing inflammation.

3) Non-Steroid Anti-Inflammatory Drugs (NSAIDs) are quite useful to reduce inflammation and back pain. You need proper prescription from your doctor. Application of local analgesic gel and oral muscle relaxants will also help.

4) Exercise for Back Pain : It is a key element of almost any lower back pain treatment plan. You may contact with Physiotherapist regarding this.

It includes 3 components: aerobic conditioning, stretching, and strengthening. The exercises are done through a controlled, progressive program to build more strong and flexible spine.
In case the pain is persistent then do consult an orthopedician for proper check up and prescriptions.
I hope this answers your query.
In case you have additional questions or doubts, you can forward them to me, and I shall be glad to help you out.
For future query, you can directly approach me through my profile URL http://bit.ly/Dr-Praveen-Tayal
Wishing you good health.
Regards.
Dr. Praveen Tayal.

---