I had it when DD2 was about 2 months old. No idea why! I was amazed how fast it came on. I went from perfectly fine to shivering in bed with a fever of 39.2C in an hour. I fed all that evening, rested, had paracetamol and fed overnight as usual. By the morning I was much better. It sounds like you had an infected fissure so probably did need the antibiotics, but for future reference, the guidelines are that it can often be cleared without antibiotics, and you should try with massage/paracetamol/warm showers and frequent feeding for 12-24 hours first. https://cks.nice.org.uk/mastitis-and-breast-abscess#!scenario
Get well soon! I couldn't believe how ill I got so quickly, but I also couldn't believe how fast I got better so hippie you'll be better soon.