How can you tell the diff btw just engorgement ...

How can you tell the diff btw just engorgement and mastitis? Baby's been a bit funny with feeds yesterday adn today so I had a sore full boob last night and the night b4. She didn't feed at all last night so I expressed that boob b4 i went to bed. Still woke up sore but other boob was fine even tho i didn't express (she would usually have both last thing). Now. That boob is fine but the other one is very sore (got a bit full by late this morning) but she has had what felt like a decent feed off it. But it's still bloody sore. Feels like a big hard lump. Very sore. Not hot. I feel fine. She is ~10 months. Am I getting Mastitis? I hope not? :-( Hoping she has a good feed tonight from both and they go back to normal.