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.

Yes: just simple muscle spasm ,it should go away . May follow workout or extreme physical exhaustion.