I already answered this this morning
on another forum I just made the case for bar-ends on riser bars. Look it up, good reading.
Pick up a bucket full of water, let it hang with your arm straight. Hard on your fingers from gripping after awhile, but not so hard on your arm, since it's just hanging straight.
Now pick up the bucket and bend your elbow, lots harder on your arm muscles.
The combination of a riser bar, which sweeps the grips back towards you, and a silly short stem like you got, puts the grips much closer to you than would a typical low flat XC bar on a long low stem. When you're climbing with the XC setup, your arms are pretty straight so they're not straining to hold your position while countering your legs' efforts. With your setup, your arms are bent because the bar is higher and closer, so when pedaling hard, especially uphill, your whole upper body and arms are tensed to hold your position on the saddle as your legs try to push you backwards.
The solution: Add bar-ends, so you can resist your legs with straighter arms. A longer stem wouldn't be a bad thing either. Your current bar and stem have you all hunched up on the bike, rather than stretched out like you should be.