It just felt like a painless muscle spasm behind my left breast. it lasted just a second but set off panic. possibly anything dangerous?

See that you suffer : From anxiety disorder. It may have been a muscle spasm since your pecs are deep to the breasts. It sounds like it only lasted a second - so you are most likely fine. If you can find a quiet place - do so. Breath in slowly ; deeply through your nostrils. Fill your lungs with air (but not to the point of discomfort). Hold for a count of five then slowly begin to exhale through an open mouth. Repeat.