Quite possibly a seroma which are really common. A seroma is a collection of fluid in the wound site which may resolve on its own or may require aspiration ( someone sticks a needle in it and drains it, sometimes multiple times )
Anything that is hot or sore, however, is to be taken very seriously. You do not want an infection and if you have one it needs to be treated ASAP.
Breast care nurses are usually better at this stuff than surgeons--if you've got one, tell her/him/them. They will escalate with your GP.