It may still be too small. You'd be surprised, what you think is the right size may not be for the snake. Should follow the general rule of 10-15% of the snake's body weight to make sure. Besides, feeding a mouse, I have a hard time believing that you couldn't move up a notch. Mice are small.
You may be feeding the right size, but eating backwards normally does indicate the snake wants larger prey. This is because the butt of the rodent is a lot larger than the head when they start on it.
The digestive system is not like the reproductive system of a human in any means, and a snake has an extremely elastic neck. Don't worry so much. The limbs are not sharp or anything.