Oops just typed a long reply then realised there are 2 threads on the same subject! Here's my (slightly amended) reply again. OP I suggest you ask MNHQ to delete the other thread.
There are plenty of cats who successfully have litters at that age without the issues that yours has had, and also manage to raise the kittens. I would suspect that she may have an illness that could have impacted the kittens and/or is making her so unwell afterwards, or possibly a nutritional deficiency. She also could have an infection e.g. mastitis which affects the milk ducts. THis leads to red swollen teats so fits in with one of her symptoms.
Sad to say an awful lot of people get cats thinking they can just feed them anything, don't get them immunised against illnesses, if indoor only don't neuter (as in this case) or take enough care to keep the animal in/keep others out (again as in this case). I know someone who rehomed a cat from a neighbour and was assured the cat was vaccinated, neutered, chipped, and a couple of years old. Cat was around 9 months, no vet care in it's life at all. Not saying this is common, but people will say almost anything to get rid of an unwanted pet.
You need to ensure she's eating plenty of high quality food e.g. kitten food to help her recover. She's following you around for comfort. If no kittens survived, she needs you to reassurance and also because she will be confused as to what's going on - this even more so if she's younger than you've been told.
Get advice from your vet on how soon you can take her in to spay. If you have papers with her check she has immunisation dates and if not take your vet's advice on how soon you can start a course. Also get them to scan for a chip, and get that done if she hasn't got one - I have indoor only cats and I'm exceptionally careful they don't get out, but I still have mine chipped, immunised etc.
Hand holds to you and paw holds to her that she recovers.