I had breast pain and redness but felt fine otherwise. I would call your GP, I think.
You can get mastitis at any time, it just is way more likely while BF and even then more common in the earlier days. There was a poster recently had weaned some time ago and got mastitis. Feel better soon!
I had fevers, chills and red streaking on my breast.
It sounds like you just have the flu though. Real flus come on fast. I haven't had the flu many times, but the two times I remember I felt totally normal and had a high fever and felt like death within about 2 hours. It's the wrong season normally, but swine flu went around in 2009 in August, so it happens.
Since you can get the fever down to a decent level, I'd got to GP in the morning. A GP could definitely help mastitis if needed too.