Why does my left breast get hard when I'm sexually aroused?

Of the three answers that precede mine, I agree that Dr. Kovanda's assessment is likely closest to what may actually be going on. Ducts do not "clog" and "unclog" with sexual activity, but you may very well be experiencing asymmetric pectoralis muscle contraction on your left side with less or no muscle contraction on your right. Swelling or engorgement during arousal does not immediately "go away" back to "very soft and normal." Muscle contraction, in contrast, can do just that.Try this experiment: intentionally contract your pectoralis muscles (try pressing your palms together with your hands at waist level. If you made fists while doing this it might be called a "muscleman" flex). See whether one side contracts more than the other. If your left flexes and your right doesn't (or doesn't as much), then you have your answer. The motor nerve to your right pectoralis may be stretched, irritated, or bruised, and therefore is not sending nerve impulses to the muscle like the left side does. As the nerves heal, this contraction will become more equal on both sides, and less like a "one-sided charley horse."This left-sided "hard as a rock" sensation should diminish and become more symmetrical over time, but it may take 6-12 months, and may never be exactly the same. Nipple sensitivity is related to sensory nerve stretch and/or irritation. This too should even out over time and is unrelated to the firmness issue, but may be similar issues with different nerves. I wouldn't worry about this, but you should review this with your surgeon at your next visit. BTW, it's not that sexual activity is to be avoided, it's just that this just happens to be the exertion that has triggered your noticing this asymmetric muscular contraction.Best wishes! Dr. Tholen

It likely has to do with the breast ducts not functioning properly and the mechanism getting clogged up, I have seen it before usually settles with time

This is an interesting question.  If you go back and read  Masters and Johnson's responses to sex, you will read  that all sexual tissue engorges with blood during sexual activity.  The real question is why your second breast isn't doing the same thing.

Discuss this with your surgeon.  In general, this could be a result of the breast ducts not functioning properly.  This condition usually settles over time.  Best wishes!