Pain that lasted a few seconds in left breast and upper back towards the left felt like it was pulsating. (any help greatly appreciated, thank you!)?

Probably not serious: Very difficult to answer a question throughly with so little information provided. Generally speaking, any pain that lasts a few seconds is not serious. If it persists, you should see your primary doctor.

Prob OK, Get checked: Have you been examining your breasts regularly as (prob) advised by your doctor? The reason is to detect any knots or lumps as early sign of breast CA, which is the most concerning question about breast. Most breast nodules are however benign but can cause symptoms like sharp pain etc and need to be examined.