Well, she hasn't been gravid for three months. That's too long even for mollies. But it's possible that she's been gravid a couple of times during those three months, and either had the fry which were eaten, or reabsorbed the fry for some reason.
Unless it's a balloon molly? Is it a balloon type? Do you keep her with males or any other fish? Do you want fry or would you prefer not to have them?
If you can share photos, and tell us a little more about your set up and what you want to happen, we'll be able to give more detailed advice