Pelafina, yes, sounds like mastitis, I'm afraid - I've had it twice now and the hot/cold flu thing is definitely a symptom. Can you get to the GP today? You might need antibiotics (I have, both times), in which case you need to start on them asap. Paracetamol can help with the flueyness in the meantime. Oh, and keep feeding from that side - it's not pleasant and your lo might get a bit annoyed if the flow slows a bit, but it really does help.
Hope you feel better soon