Brief Answer:
Due to muscle strain -

Detailed Answer:
hi
Thanks for the query
I can understand your concern.

In my opinion your back pain is due lower back muscle strain. For pain you can take pain killers like tablet ibubrofen, Acetaminophen.

If pain is too much to bear you can go muscle relaxants like tab ultracet along with Tab Pantop 500mg for 2 days.

Follow below tips to reduce pain
1 hot packs/cold packs over pain area
2 hot oil massage to the pain area/physiotherapy
3 Watch your posture.
Be especially careful of your posture when lifting heavy objects. Never bend over from the waist. Instead, bend and straighten from the knees.
4 Watch your weight. Use diet and exercise to keep your weight within a healthy
range for your height. Being overweight puts excess stress on your spine.
5 Rest your back while sitting
6 Avoid longstanding and sitting in same posture
7 Avoid smoking , it causes osteoporosis ( bone weakness)
8 Avoid 2 wheeler ride for long distance.
9.You can start calcium supplements if required
If it is not reducing with above methods you may need to meet Orthopedic surgeon

Thank you
Take care

Regards
Dr Priyanka XXXXXXX

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