My LO is almost twelve months old and over the weekend I started to come down with a cold and also noticed that I had a particularly tender and sore breast (almost like it was bruised). I was definitely feeling fluey which could indicate mastitis but I couldn't feel any hot spots or hard / plugged ducts in my breast. By Monday I was feeling much better and the tenderness and pain had gone (but my LO had contracted my cold and croup!).
Is it possible that the pain in my breast was "glandular" as such and just related to my cold or is it possible that my immune system kicked in and knocked out the potential mastitis?
I don't think there's any way to tell for sure what went down. But because your LO is now sick, it suggests that what you had was a virus, and the sore breast was a coincidence.