There can be a number of causes for middle/left back pain. The most frequent cause is muscle strain or muscle spasms. Since you said the painful area feels tight, that's a good possibility. Other common causes of pain in that area can be discs that are beginning to deteriorate or arthritis.
You mentioned that it happens more often when you're sitting at work. If possible, try a more ergonomic-friendly chair and/or try to stand up and gently stretch or walk around for a few minutes every half hour or so. I've found that sitting for long periods of time in one position can cause muscles to tighten up and be painful.
Other simple things you might try include using ice packs or heat packs – whichever seems to help the most, taking an anti-inflammatory medication like ibuprofen or aspirin, or applying a topical pain reliever.
If you think it's a muscle strain or spasm, a massage therapist should be able to help you. He/she can also show you some exercises that will help relieve tight muscles.
If none of these things help or if the pain gets worse, it would be good to check with your doctor.