You need a better stance before you shoot. Lean forward with a foot forward and 70 % of your weight on the forward leg and it won't do that.
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Guns scare me. :)
With your petite frame, you would probably do well with a BullPup shotgun.
Attachment 36789
Attachment 36790That is a Keltec KSG...kind of spendy.
But they make a bullpup kit for a remington that is a lot more affordable, and a lot more reliable! This pic is a before and after conversion:
She actually shoots the gun pretty well - leans back a bit in the beginning, but she has some technique to improve. She is too low on her shoulder....pretty common as people think the entire pad of the buttstock should be in the shoulder. And she is chicken winging a bit, which allows the shotgun to move out a bit. There is a nice little pocket just to the inside of your shoulder that the bottom of the shotgun should sit in, it allows great contact that bruises minimally, and you don't have to drop your head to get on the sights (as much).