I've been having muscle cramps into the left side of my chest/left breast. Are these coronary muscle spasms?

No. At your age, very unlikely to have coronary muscle spasm. Your heart is normal, unless I am not aware of a heart condition. However most likely this is an intercostal or rib pain in the area. Also could be do to muscle spasm as well.
Unlikely! Does not sound like typical heart pain but one should have more history (age, duration, circumstances of the production of the pain, other medical conditions etc ! ).

Related Questions

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. Read more...

My left breast just started twitching um muscle spasm or something more serious? I could see the twitching

Probably not serious. This is likely a fasciculation, localized muscle twitching which is usually not serious, esp. if it happens rarely. It can be related to deficiencies of calcium, potassium and esp. magnesium, so supplementing with magnesium, esp. as mag. taurate or glycinate is worth considering- see http://drcarolyndean.com/magnesium_miracle/ Stress may also contribute to fasciculations. Read more...