Go to a chiropractor. The popping sound indicates that the bones have been pulled out of alignment. Your tense muscle will be continually pulling this muscle out of alignment. The chiropractor can manually relax the muscle by applying pressure, and then put the bones back in alignment.
If you wait the pain might subside after several days but who knows, the bones may or may not move back into alignment. Subluxation of the spinal column can cause nerve damage, and perpetuate further problems in posture and muscular imbalance.
If it's your first visit they will likely need to take an x-ray first, or at least a thermal reading.