Are you standing bolt upright or even leaning backward some, instead of slightly leaning forward, into the gun?
If so, that will cause your gun and arms to go up.
As will extending your arms too far out.
Your elbows should be bend enough to bring your arms and the gun back enough so the shoulders support them instead of the shoulders merely acting like hinges.
That allows the recoil to come back instead of up.
Also remember to bend your knees a little, to act like shock absorbers.
Take a look at the guy in the picture at the top of this page: