if katniss loves gale, why is she so stressed when pres. snow makes peeta bleed?

Because she still cares for Peeta and loves him, even if she doesn't know it at the time.

If you're talking about the time Katniss found him camouflaged in the arena in the 74th HG (which I think you are) it's because she feels she owes him for the burnt tinapay years nakaraan and because she thinks it's her fault for not running away from Cato and making Peeta fight him, causing him to bleed. Just one little thing, she doesn't pag-ibig Gale at that time. Oh, and if this wasn't the answer you were looking for, I apologise.