why did my cat suddenly die

my name is kathy and the cat was called poppy. she was 2 years old and just 5 weeks ago had kittens. yesterday morning she was her usual lively self, i had popped out to see friends and when i came back she was lying in the outhouse struggling to breathe, she could not meow and could not use her hind legs.

i rushed her to the vets they put her on a drip and gave her antibiotics, about two hours later she passed away please can you help me and tell me why this happened. as i said she was fine in the morning. one more thing aswell it seems like she could not see properly either as when i went to pick her up she bit me.

i am now worried about her kittens, one of them seems to showing similar signs.

What answer did your vet give to these questions?
 

thye said it was an infection but could not say anything else unless i pay for more tests to be done

It sounds like this is a vet you are not used to seeing otherwise they would give you a little more information, at least some possible scenarios. Can you afford to take the kitten to the vet? On face value, thinking only of Poppy, they probably thought a simmering infection from birthing, but may have different ideas now they know a kitten is sick.

Are you saying the kitten is struggling to breathe, can't use his/her hind legs and seems to have vision problems?
 

it was breathing funny and if you move your hands in front of his eyes he does not react

Due to not being able to use her hind legs, I would say it might have been a saddle thrombosis.  If so, there was nothing you could do except get her to the vet.

Is htere any way that Poppy or her kitten could have got into anything poisonous? Some poisons cause blindness and paralysis.

well to be honest she didn't really go further than the back garden and never went out the front so i cant see that being the problem. i have read up about heart conditions and her symptoms are similar. the kitten is fine now though i think it was just me panicking because of poppy