Well, the way i see it, they aren't flipped, but just rotated, as you rotate the whole sprite, not just the speed vector.
If you want to change the movement direction by 180 degrees, you could either:
1.) set the speed to a negative value, for example
"when sprite at necessary position / collides with the border -> sprite(bullet)speed = sprite(bullet)speed * (-1)"
2.) rotate the sprite by 180 degrees and flip it vertically, you'd do this by setting the height to a negative value
"when sprite at necessary position / collides with the border -> spriteheight = spriteheight * (-1)"
horizontal flipping could be necessary, also.