Yes their behavior normally becomes different. They become very affectionate and lay around with their butts up in the air. They cry like crazy and will try to escape outside to mate.
You better get her fixed as soon as possible. Since you didn't post in the breeder forum I assume you are not a breeder, especially since you were asking about signs of heat
So spay her right away before you have an accidental litter to care for. If she is under a year, she certainly should not have babies because she is not mature enough.
If cost is a problem you can do a search in your area for a low cost clinic. Either an internet search or call your local shelter or humane society and ask if they know of any.