The breasts have very sensitive tissue. It’s possible that the bruising may have been caused by you bumping into something (you might not have been aware of it at the time) that caused you to bruise. In the process, you might have broken little blood vessels that have bled causing bruising.
Most symptoms have straightforward medical explanations. See how things go over the next week or two and if the bruising is not showing signs of subsiding do contact your GP to have it checked out just to be on the safe side.
