Pregnant stray

So I recently took in a pregnant cat that the previous owner said she had already had one litter (only two) both of which passed away after only 48 hours. The girl said she couldn't take care of her anymore so I took her in and now I believe she is pregnant again. Which must of happened before I got her because she is strictly an indoor cat now. I've been feeding her vitamins to help with this pregnancy but I'm worried these kittens will pass too. Vet checked her for feline luk. And she's utd on shots, but she is very small. Vet believes she may just be a smaller cat or the runt of her litter. She will be a year old in June and has already had one litter has anyone else had experience with young mothers? I will be getting her spayed as soon as kittens are weaned as well.

Kittens passed away because they were outside the first time. And were born in January.

Also should I put her in one room when she has them? Or let her roam?