My name is Scott and I am an experienced small animal veterinary surgeon, I will be very pleased to work with you today and will try my best to answer your question to your satisfaction.
This is not good, it is possible that she has some form of infection related to giving birth such as a womb infection or perhaps she has retained afterbirth
I wish I could resolve the matter for you via the internet but it is very possible that prescription drugs such as antibiotics will be urgently required.
As there is really nothing you can hope to do on a home remedy basis my advice has to be to have a vet examine her without delay.
the fact she gave birth two weeks early could be related to this, and one week after giving birth cats could be a bit immune depressed and so more open to infections.
I hope I have covered your question fully enough but if you would like further clarification or to talk things over a bit more then I will be on-line for the next hour or so and I will be more than pleased to continue working with you.Regards,Scott