I lost 10 lbs the first month I had Magnus... I even slept in his "room" few times b/c I thought he might get sick. I was little more laid back with Zoey but her being so much smaller did made me nervous in other ways. And again, I slept in their room few times to make sure she was okay after any type of incidents.
I pretty much just kept telling myself that s/he will be ok. And as they got older and stronger, my stress level went down as well. I think once you both go through the stages of first year vaccines and spaying (if you're not planning on having litters), you'll be a lot more relaxed.
Try to relax as much as possible. They can sense things like that...