Hi, no. This is not a risk for pregnancy. You can't get a girl pregnant by fingering her. A penis inside her vagina is how she will get pregnant. Do not ever do that without a condom on. The penis can give off sperm before full ejaculation when inside the vagina so this is why you shouldn't even put it in without a condom. But your fingering your girlfriend is not going to lead to pregnancy. And please don't call her fat. All women gain and lose weight from time to time. Hormones can fluctuate and it can lead to weight gain during even their one month cycle. They can gain weight when they are happy and not trying to micromanage it. And your goal is to be encouraging to her so she feels good about herself. But pregnancy is not a concern here.
I might also add, since the time span you are describing is February 27 to March 6, women who are pregnant are not a visibly different size in only a week.