Several things can cause this, as mentioned stress, like being picked on, chased, heat etc. Also possible she has a tight egg duct, where one egg is in place and the egg behind it bumps into it as it comes down the egg duct. Keep a close eye on her as it is possible she may become egg bound. Saw this as a kid, grew up on the farm with 100 plus chickens so it was next to impossible to determine which hen it was till it got sick, or died.