PLEASE HELP ASAP I THINK MY NURSING CAT IS ILL NEED ADVISE BOUT THIS INFECTION?

When i came home from work and was stroking my Siamese Angel. Angel has just started to wean off her kittens and they have all new homes, but due to our old Tom Siamese Prince ping, we have kept one of her kittens Missy, who tends to suckle on her mother - Angel does not seem to be in any pain and happily purrs as missy suckles away. Missy is 12 weeks and has all her teeth and claws but yet again angel seems happy and content when feding. Anyway i noticed that one of her lower teats were swollen, she has had a litter of kittens (5th litter) and is 5 yrs old, she has never had any infections or swellings like this before so i wasnt very paranoid but made an app with the vet anyway and his earliest was in 4 days i took the app anyway but today i have noticed that the swelling has grown and spread not only to the areas around her nipple but through thher inner abdomen, I ume its mastitis but i have to wait till my vet app on Monday afternoon. What do you think, and s mastitis fatal - please only say "yes" it is if u know what u are talking about as i dont want to be worried for no reason. Thankyou