Apparent brokn blood vessel (red streak 3-4in long) on top of breast with blood pooling under nipple (2in dia / tender) 27 yo female. cause for alarm?

No, but...: In absence of at least a photo it is not easy to make an accurate diagnosis. There is a benign condition called mondor's disease. Again it is benign. It is a clot i.e. Thrombophlebitis of a chest wall vein. Can be red and painful. Any anti-inflammatory drug can be helpful. Have however an obgyn see it. Btw, the name "broken" blood vessel is murky to say the least...