Visit a orthopedic doctor and go for spinal investigations and MRI spine, meanwhile take pain killers and muscle relaxants. You can also try baclofen. If you find this helpful then give vote of thnx.

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Brief Answer:
I would suggest an x-ray and the following...

Detailed Answer:
hi, thanks for your query. Welcome to HCM.

You stood up from awkward position and you felt shooting pain in lower back. You have shown to doctor and he has said that it is a muscular spasm or sprain. You have taken muscle relaxants and other medications but it is not improving.

Now, It is a time for investigations of lower back like X RAY lumbosacral spine. After that you have to show this to orthopaedic doctor or your previous doctor.

Mean while avoid lifting heavy weight, working in bent position, avoid squatting position, do not sit by folding legs. Visit a physiotherapist and learn some simple exercise to strengthen muscles of lower back but this should be after your pain has gone.

If there are any abnormalities in X RAY found, your doctor may advise you for MRI spine. Also you can start supplements of calcium up to 1000 mg in a day and vitamin D3 satchet one per week for 6 weeks. this will strengthen your bones.

If you spasm has been there for more than 7 days then there is no need to wait for long and visit a doctor immediately.

Hope I have answered your query. if you have further query then feel free to ask. Wish you good health.

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