If she is a Molly, she probably is! If you just got her from the pet store she is probably already preggers. If you have male Mollies in the tank then DEFINITELY. Mollies are the rabbits of the water world. They breed efficiently.
I bought my Molly and took this picture and less than 2 days later we had 14 baby fry!
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Then, 34 days later she had 7 more. We have no males and all the babies were in the breeder net. They can give birth to sets.
Keep an eye out and best of luck!