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Tetras don't get pregnant, they're egg layers. It's possible she became what's known as eggbound but it's unlikely. My guess would be dropsy, which is a sign of internal organ failure. Dropsy has many causes, and is usually fatal. This link has some info on trying to treat dropsy:
Flippersandfins.net on treating dropsy!
What I've seen sold as fruit tetras are usually dyed fish. They have a shortened lifespan, and are often unhealthy.