Welcome to the forum!
To begin, Tetras don't have live fry so she can't be pregnant. She may, however, be full of eggs. If she is, then in due time she should drop them. Are her scales sticking out, like a pinecone? If so, the tetra may have dropsy. There is no cure for dropsy as it is a symptom of internal organ failure and by the time it shows it is too late. The third possibility could be that the tetra is constipated. Try feeding your fish shucked peas that have been blanched (boiled in water for a couple mins.) Hope that helps!