Like its been said already, the gun doesn't fit or when firing, you aren't mounting properly.
This is probably too hard to explain properly without physically showing but I'll give it a go..... A very basic way to gauge if a stock is too long or short is to place the butt into the hinge (elbow joint) and place your finger onto the trigger. A correct fit is when your finger reaches the trigger and you are able to place your first joint of the trigger finger on the trigger. If you are able to wrap your finger around the trigger completely its TOO SHORT and likewise, if you have to stretch your forearm to comfortably get your finger onto the trigger its TOO LONG.
This is a very simple and crude way of checking but I would advise you to go to a shooting ground or good gun shop and get them to check the guns fit.
Another factor may be cartridge choice, if you are shooting a heavy load it can very often make the gun 'bounce' thus bruising your face. I had that problem with my Beretta Ultralight, I've had the gun checked by a shooting coach and it fits me perfect. I can shoot any cartridge load up to 30g but anything over that knocks the s*** out of me.