Ok. Thank you for the additional information.
From what you describe I am gathering that you have mainly left sided lower back and buttock pain, you are struggling with walking and climbing stairs, but are comfortable sitting and lying down.
This could be resulting from nerves getting pinched on your spinal canal....medically termed as neurogenic claudication.
If the symptoms are acute (<6 weeks), then the treatment is predominantly conservative. Pain killers (Tramadol and NSAID such as Ibuprofen if you can tolerate that), rest, physiotherapy, local heat treatment, etc would help.
If symptoms have been present for > 6 weeks, then this needs investigating further with an MRI scan for sure.
You will need to visit your GP and get referred to a Spine surgeon for the same.
Very rarely these symptoms can be secondary to hip arthritis too and this can be ruled out with a simple hip X-ray.
Hope this information is helpful.
Let me know if you have any further questions.