How do I know if I have mastitis?

F

I have what I thought yesterday was a blocked milk duct. I've had them several times before, but usually after massaging the sore area during a warm shower and a couple of feeds it clears up. But this time is hasn't. It's now the end of day two with this terribly sore, hard spot on my right boob under the nipple. I'm not showing any other signs of mastitis I don't think, like I don't feel unwell, but I'm afraid it will turn into mastitis if I can't clear the duct soon... What signs of mastitis should I look out for? And any tips on preventing it/getting rid of this blocked duct? It's sooooo painful 😢