i would go to the doctor in the morning if you can - i generally steer clear of antibiotics if there's any way of avoiding them, but mastitis can hit you very suddenly and worsen quickly, so i would get on with the antibios sooner rather than later. i've had it a few times and only once got the red marks.
Ive had mastitus twice recently due to my dd being a slow feeder and its awful. You will know when you have it and I would suggest you have the start of it and it could develop. You can massage your breasts especially if you find any lumpy parts or they have red streaks on the skin or if its tender. If you can get rid of the blocked duct it may keep mastitus at bay. And get plenty of rest and drink lots of fluid and if it develops still then get the antibiotics and you will feel better in 24 hours. Hope you feel better soon.