Pressure in lower back and left neck while sitting only, but no pain while walking or exercising? very dull, not sharp at all and happens sometimes. i work in office and sit for a long time sometimes?

Postural strain is a very common reason for neck and back pain. You need to be seen in person to evaluate this.

Sitting for a long time and poor posture may be the cause. You may try to get a standing workstation. There are devices that can go up and down so that you can work sitting and standing at intervals. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs.