First of all, if you're a nervous mom, I wouldn't worry about bothering your vet. That's what they're there for! You love your kitties, and they're very, very lucky to have someone to worry about them. Cats are so good at hiding the fact that they're sick or in pain, and it is often only through slight changes in behavior that we can tell there's something wrong. So until you're real familiar with them and their behavior, it is always best to be safe rather than sorry!
I am unfamiliar with the reasons for empty patches of hair. Sometimes cats can have a obsessive/compulsive disorder where they lick it out themselves. This does not sound like that! It could have been lost in a scrap with another cat, though that would usually be accompanied with some scratches or sores on the skin. This doesn't sound like that. Cats do have skin diseases, but if she was examined by a vet and got a clean bill of health, then I'd likely rule that out (because that would likely be accompanied by flakey skin or something).
If the hair is growing back in, I'd just keep an eye on it. I'd watch the appearance of the skin. If there seems to be no issues with the texture of the skin, and there appears to be nothing underneath the skin - no bumps or bruises, etc., and she's not obsessively licking it and the hair is growing back in, I'd just keep an eye on it.
If you adopted them from a breeder, I'd call and ask about it. If you adopted them from a shelter, it might not hurt to call to see if they have any health records for her - perhaps her records would note something.
And if you're still worried, I'd take her back to the vet. We don't EVER hesitate to call or make an appointment any time we ever have any questions about our kitties' health. Of course as we got to know them better, things we didn't understand before we don't call on now.
Glad you found TCS!!!! Hubby and I didn't know ANYTHING about cats. We didn't even intentionally get involved with cats! Our relationship started with them when a stray turned up. Now we rescue cats and have five living indoors with us. This site was of immeasurable help in answering so many questions about their health and behavior - and I found it to be just a great community of people. PLEASE feel free to browse through the threads in especially the health, behavior and grooming forums. You'll be surprised at how much there is to learn!
(And the best thing about it all is the ability to get on and ask questions.