Sorry to hear that you are in lot of pain
First you need to answer a few questions....
since when are you having this backache.
You feel the ache in only lower part or the whole back.
Do you get pain even while lying down/at rest.
Did u suffer from Chikungunya of dengue fever recently??
You need to immediately get xray of your lumbar spine/spine done in lateral and AP view.
you are a young man of 28 you also need to get your serum calcium, uric acid, CBC and RA test done.
as you have mentioned that you have pain when you sit walk and change posture and it seems your pain threshold is pretty high so you immediately need proper medical evaluation ....get your mentioned xrays and blood reports done so it can give idea to the evaluating orthopadecian/physiotherapist as to arrive at a provisional diagnosis.