Hi Midna,
I had mastitis, really bad fever, felt like someone was stabbing a knife into my nipple every time he fed, shaking, etc, etc.
Feed off the infected side a lot. Massage it before you feed to get rid of blockages and lumps. ANd the hot compress before feeding also a good idea.
Also, savoy cabbage. Sounds crazy but you use the leaves inside your bra against your breast. It actis like a poultice and really soothes the pain.
Other than that, strong painkillers regularly. Something like ibruprofen is best as mastitis is inflammation and these painkillers are designed to combat inflammation aswell as pain.
If you're having probs with the penicillin, you can have a different type of antibiotic. I'm allergic to pencillin and I was prescribed a different one (can't remember what it's called now).
Hope it gets better, it's definitely a btich but hang in there.
K.xx