I have had it for about 3 months now I think. But only noticed her getting bigger and bigger for the last month or so
You have yourself a normal balloon molly. They do look like that.
If she's been anywhere near a male, including while she was in the tank she was bred in, or at the store you got her from, she could well be gravid as well. They can store sperm from the males for months, some have said even for up to a year. It also doesn't mean anything that she's been like that for over a month, they don't always use stored sperm right away, and it's hard to tell in the early stages when mollies are gravid, and it's even harder when it's a balloon molly, since they are already so round! Plus fish can look bloated after eating, or if they've been overfed.
Just make sure you're not over feeding them, and keep an eye out in the tank for tiny fry, just in case! When newborn, they tend to head for the surface, especially if there are floating plants and cover, or they try to hide down in the substrate.
Adult mollies will happily eat their babies though, so you might have fry born, and not even realise it, as the adults catch them all before you see them. Providing a lot of dense plant cover, including floating plants, and keeping the tank well fed without overfeeding, will give fry a better chance of avoiding being eaten.
