Could be some acute sprain or even a disc problem of spine. Get evaluation by orthopedic specialist who on physical tests can determine the origin of pain and whether radiological tests are required for diagnosis..

In mean time take combination of methacarbamol 750 mg and ibuprofen 400 mg thrice a day after meals,, apply hot packs, take firm mattress to lie down and maintain the most pain free posture on lying down in any position.

Hello,
Welcome at HCM.

I have read details of your question.

It looks that you have worked hard for last three days.
Standing for long time does lead to strained hamstring muscles ,due to this you had an excessive strain over the joint of back.This condition is known as sacro iliac dysfunction in medical terms.

You should not worry about this.Consult to a physical therapy specialist ,he will do some manipulative therapy & teach you some exercises.

Take a good analgesic like advil or aleeve.

Hope this all will help you.

With best wishes.

Hello,
Thanks for writing to us.
You might be suffering from Acute mechanical back pain.
1. Bed rest for 2-3 days
2.Antiinflammatory and muscle relaxant will provide some relief.
Take medicines under medical supervision only. Application of local pain relieving gel will be helpful.
3. Hot water fomentation to back 2-3 times a day
4. Physiotherapy - Interferential therapy (IFT) to your back will help you relieve the pain
5. Avoid driving, leaning forward, sitting for prolonged time or lifting heavy weights. These activities can increase your back pain.
Even after a week if your pain doesn't gets relieved, better to get yourself examined by an orthopaedician. He might ask you to take an Xray of your back
Hope this helps you.
Regards.