If she's shared a tank with a male at some point in the last 6 months, then yes she is almost certainly pregnant. She can store sperm packets for up to 6 months, and use them to produce fry drops of 30+ every 4 weeks or so.
Before you put her in the spare tank, and try to save the fry, think about what you are going to do with the fry. DO you have a reliable outlet for them? Many LFS won't take hobbyist-bred platies, simply because they are too cheap and easy to obtain from their normal wholesale suppliers.
You wouldn't be able to rehome the fry for at least 3 months, so if you're going to try to keep all the fry, that could be over 100 little fish. In a 15gallon?
Sometimes, it's best to let nature take its course, and let only the strongest survive. If you do decide to separate her, wait until her stomach looks to have very square corners, and she starts to hide herself away.