Depends if you just fed them and noticed the swelling? What colour is their poo? How long have they been swollen?
"Sexing Neon Tetras can be hard, but the female usually have a bigger and rounder belly than the male. The blue line will be straighter in males, while the round female body creates the impression of a bent blue line. When she is ready to breed, her body will become very broad since she is filled with eggs."