Hi Joanne4,
If the turtle is actually a female and if she has been kept outdoors and if she has actually bred in the past, then from your description, it sounds like she is gravid.
There are a few things you can do to find out if this is the case:
1. Take her to an experienced turtle keeper and ask them to palpate her for eggs;
2. Palpate her yourself, but this can be very difficult if your are inexperienced;
3. Get her x-rayed to confirm if she is gravid.
If she is gravid, she will need nesting sites. The best way to do this is to put her in a large outdoor pond with suitable nesting sites within the enclosure. Of course, even if she is not gravid, she will be better off living in a large outdoor pond, as opposed to being kept in an aquarium.
My advice would be to not feed her earthworms. Australian Freshwater Turtles eat underwater. Anything that lives in freshwater is potentially part of a turtle's diet. Earthworms live underground, therefore they are not part of a normal, healthy turtle diet.
Regards,
Michael.