Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about Lacey.
I am assuming that you have not had Lacey's pregnancy confirmed by her veterinarian via an ultrasound or radiographs of her abdomen. At about 45 days fetal skeletons start to become calcified so a radiograph is a good way of confirming pregnancy and letting you know how many kittens to expect. An ultrasound will allow confirmation earlier but it is very difficult to get a good kitten count with that as the uterus and kittens move as the probe is moved.
Not all breedings are successful, and even when they are not all kittens are carried to term. There can be kitten resorption for any number of reasons.
If she is pregnant she should be about 49 days along. Cat pregnancy lengths can be variable as we do not always have implantation right away and that can increase the length of pregnancy. In most cases pregnancy is 63 to 65 days in length however. At this point in her pregnancy I wouldn't expect much change except a "fuller" then normal abdomen. In the next couple weeks I would expect her to gain most of her weight, her mammary chain should begin to get full and in the last week to the day of delivery she should start lactating. So really she may be completely normal as far as physical development.
Although it seems unusual some cats do have heats during pregnancy, especially toward the end of pregnancy and will mate if given a chance. Odds are this won't be a successful breeding, and even if it is the kittens would probably be lost when the first litter is delivered. This is not considered abnormal and is not at all harmful to her, but I would not recommend letting her mate.
Alternatively some cats get hormonally affected toward the end of pregnancy and will become very, very affectionate, almost mimicking heat behavior, but won't mate if given chance. This too, is considered normal behavior.
At home all you need to do is make sure to keep her away from males, and make sure she has an appropriate area to deliver that she is very comfortable in. You may also want to consider having an abdominal radiograph done so you know how many kittens to expect and to confirm pregnancy.
Best of luck with Lacey, let me know if you have any further questions.