The most common cause of back pain at the age of around 20 year is postural or traumatic. You have mentioned in your question that you have used to sitting for a long time in same position. so it may be a cause for it or u may have history of injury in past. Back pain may be due to other cause like genitourinary diseases, functional , osteomalacia, or any malignant disease, strain, sprain, tuberculosis, vertebal fracture etc.
Yes, there is exercises for back pain.
For treatment of back pain various aspects are there.
1. Rest: In severe pain absolute bed rest.
2. Drugs: Analgesic and muscle relaxant.
3. physiotherapy: Heat therapy, hot packs,